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Electronic Fuel Injection

  Experience in a Book
Electronic Fuel Injection


If you wish to understand or troubleshoot the Jaguar XJ-S electronic fuel injection system, it is highly recommended that you obtain a copy of "Understanding Electronic Fuel Injection, Ignition and Engine Management" by Roger Bywater and available from AJ6 Engineering. The book is written as a primer on EFI in general, but since Bywater's expertise comes from years of working for Jaguar many of the examples presented happen to be Jaguar systems. Often, it seems he is talking about these cars in particular.

According to Dick White, "There is a book ëBosch Fuel Injection and Engine Management' which I found very informative. It contains many references to Lucas." It is available from EWA, among other sources.


FUEL INJECTION SYSTEM TYPES: The 1976-80 XJ-S used a type of Lucas fuel injection system based on a Bosch D-Jetronic design. From 1980 on, the cars used a system referred to as a Digital P, also similar to Bosch. The two systems are completely different. Among the differences: The earlier D-Jetronic system used a trigger board within the distributor operated by a magnet in the rotor; the later Digital P system simply derives its triggering from the ignition pulses. The D-Jetronic has a "throttle switch" in the bellcrank on top of the engine that provides an on-off pulse about ten times as the throttle moves from idle to full open; the Digital P has a "throttle potentiometer" in the same location, providing a smooth varying resistance as the throttle moves. The fuel regulators on the D-Jetronic system maintain the pressure in the fuel rail at a constant value, while the regulators for the Digital P system vary the fuel rail pressure according to intake manifold vacuum.

The D-Jetronic has no oxygen sensors, lacking the sophistication for feedback control. The US-spec Digital P has two oxygen sensors, one in each exhaust downpipe, and Roger Bywater reports that the ECU will control the mixture in each bank separately. The Digital P systems in countries without strict emission laws had the feedback capability omitted from the ECU, and had no oxygen sensors.

Bywater elaborates on the various systems: "D-Jetronic ran from 1976 to 1980 in the XJ-S but, just to confuse things, ran until 1981 in the saloon. Digital P 6CU first appeared in July/August 1980 on the 10:1 compression flat head engine rated at 300 b.h.p. in the XJ-S. Most XJ12 saloons continued with the earlier 9:1 D-Jetronic engine until the arrival of the H.E. engine in July 1981 but a few saloons were fitted with the 10:1 engine and Digital P. I understand that the improved fuelling accuracy of the Digital P EFI system was what made the higher compression engine viable.

"The 10:1 engine performed very well and was certainly the most lively version of the 5.3 ever produced but there were two things which could make it a bit fragile at sustained high speed. Firstly the cooling ability of the radiator was a bit marginal, secondly the distributor build quality was not too good and we measured errors of up to 10 degrees excessive advance at 6000 revs, which is obviously likely to cause trouble. Not surprisingly, there were tales of piston failures on German Autobahns.

"Those early 6CUs triggered from the coil negative terminal and the ECU was protected internally from the high voltage firing spikes by a 100K resistor in the input circuit. This was not necessary with the Constant Energy system introduced for the H.E. and was replaced by a link. Another important difference, apart from the fuel requirement of the 10:1 engine being different from that of the later H.E., was that full load fuelling was set into the mapped fuelling values so there was no need for a separate full load enrichment switch circuit. This means that fitting an ECU intended for an H.E. car to one of these engines is likely to magnify any tendency to burn pistons because the full load condition will be excessively weak. A number of people have found this out the hard way. Strangely the ECU seems to survive the high voltage firing spikes despite not having the protection resistor.

"The H.E. engine arrived in July 1981 in both the XJ-S and XJ12 saloon so that is when D-Jetronic really ended."


INJECTOR GROUPING: The wiring harness is set up to operate the injectors in four sets of three. However, within the ECU these circuits are combined to form two sets of six. Reportedly, in the early D-Jetronic systems, each set of six injectors includes three on each bank. However, in the later Digital P systems (which include oxygen sensors), each group includes all six injectors from a single bank, presumably so the ECU can control the mixture of each bank as a group.


XJR-S: John Goodman reports that the XJR-S has Zytek ignition and sequential fuel injection. "It is essentially the same injection system off the '80's group "C" Le Mans race cars, Jaguar made a big thing about publicising it. But I suspect TWR had a load of spare bits left over and needed something to do with them!"

"Also there are two power resistors on the R/H front inner wing.

"I thought Jaguars claims for improved fuel economy was bullshine, but it is more economical than my previous H.E."


EFI GROUNDING PROBLEMS: Michael Neal sends this experience: "...the radiator had been replaced and a ground for the fuel injection harness at the right front of the engine compartment had not been re-secured. It had blown two ECUs before I found this. I don't know why but it took the ECU a week or two to blow. After I replaced the ground and ECU there were no further problems."

Reportedly, if the engine clearly is running very rich or possibly won't even start because it's too rich, it's time to start looking for grounding problems in the EFI wiring. John Napoli relates one experience: "Although ground to the ECU was one of the many things we had checked early on, the owner later went further and checked the ECU connector. He found an intermittent ground on pin 1 at the connector. This pin, according to our schematics, should ground to the frame of the car at the ground points near the battery. This wire was identified in the loom and a splicer to ground added. The car started fine and at this point is OK."


ELECTRONIC CONTROL UNIT: The following is a list of the ECU part numbers, hopefully mostly correct. Thanks go to Jeffrey Gram, AJ6 Engineering and other sources.

D-Jetronic: Type 3CU LRZ103 and LRZ113?
Digital P: Type 6CU catalyst version: DAC 2597 and DAC 3586
non-catalyst version: DAC 2596 and DAC 3062
Type 16CU catalyst version: DAC 4118, DAC 4585, DAC 6335, and DAC 6337

These are the low compression version - presumably 11.5:1

non-catalyst version: DAC 4119, DAC 4478, DAC 4586 and DAC 6336

These are the high compression version - presumably the 12.5:1

Additionally, there is a low compression non-cat version: DAC 6338.

For one year (1981), the XJ-S had Digital P EFI on a pre-H.E. engine. Clearly, the fuelling maps for such a car would differ from those for the later H.E., and the ECU's would not be interchangeable. It's not known if any of the part numbers above apply to the pre-H.E. Digital P.

Note also that the Marelli ignition system provides a cleaner signal to the EFI system, so the ECU's fitted to Marelli-ignitioned cars have less filtering in the signal input circuitry. This may help account for the profusion of 16CU part numbers listed above. The pre- and post- Marelli ECU's are not interchangeable.

Richard Mansell quotes from a Jaguar publication on the changes for the 1992 model year:

The new fuel control system, the Lucas 26CU, replaces the 16CU and provides the following benefits:

- Improved starting performance at all temperatures.
- More efficient engine operation during warm up.
- A diagnostic facility which stores fault data for later retrieval.
- Low quiescent current drain.
- Eprom socket to allow modifications.
- JDS diagnostics serial link.
- Fuel used output.
- Enhanced software.
- Enhanced limp home capability.

With the pre-facelift ECU, (Lucas 16CU), one injection is delivered per engine revolution. In the facelift condition, the Lucas 26CU, there are three injections per engine revolution. The number of pulses is reduced to one per revolution after a set number of injections.

After-start enrichment -- The enrichment is now coolant temperature dependent. It decays away over a number of engine revolutions instead of over a time period and is no longer linked to cranking fuelling so that better calibration is achieved.

Acceleration enrichment is reduced; calibration is improved; over enrichment is bettered in the occasions where the driver opens the throttle very quickly.

Full load Enrichment -- Where full load enrichment was determined by vacuum operated switches sensing inlet manifold depression, it is now determined by throttle potentiometer position and engine speed.

There is a single 2.5 bar mini fuel pressure regulator instead of two.

The people at John's Cars say they will test your ECU for you, free of charge. They are confident you will eventually be buying something. They add, however, that since testing requires a car, there may be a few days' delay until they have a car with the correct system on hand for the testing.

Of course, you can probably learn a lot yourself if you have a friend with a car similar to yours. Swap the ECU's and see how many problems go with them.

If your ECU gives up the ghost, please see the comments under Performance Mods before spending the major $$$$$ for a new one. Also, you might want to contact AJ6 Engineering or Corsaro Electronics.

If you wish, you can open the ECU easily enough; it is a simple cover-body-cover sandwich with several screws holding it together. Note that a couple screws have aluminum caps that serve as a tamper indication; it might be wise to inquire about policies regarding exchanging an opened unit before opening.

Take precautions to avoid static electricity discharges while inside this unit. Usually, it is sufficient to keep one hand on the chassis while working.


ECU INTERCHANGEABILITY: The Lucas 6CU and 16CU ECU's are "pin-compatible", meaning you can replace one with the other without making any other changes -- provided that everything else in the car is the same, such as engine compression. Don't try putting an ECU from a 12.5:1 spec car into an 11.5:1 car. The 26CU is not interchangeable with the 6CU/16CU.

Of course, the 16CU is considered a better ECU than the 6CU, so owners of earlier cars may wanna upgrade while owners of later cars might object to installing the earlier item.


EFI WIRING HARNESS TROUBLESHOOTING: Matthias Fouquet-Lapar suggests "one thing I would do is to put in fixed resistors for the various sensors direct at the ECU connector" to find out if there's a problem in the harness.


FUEL INJECTORS: If your engine is running or idling rough, one possible problem is a plugged or bad fuel injector. This may be indicated if the misfire always seems to be the same cylinder, although a bad spark plug or wire or mechanical problems in that cylinder are other possible causes. One-cylinder problems have different symptoms than they would have on a carburetor system, because a misfire in one cylinder causes the fuel and oxygen to enter the exhaust unburned. The oxygen sensor will pick up the excess oxygen, and the ECU will conclude that the engine is running lean. The ECU will then richen the mixture to all 12 cylinders, and then all cylinders sound bad, not just one.

If you have a trip computer in the car, chances are pretty good that the indicated fuel mileage doesn't agree with the actual mileage determined by the odometer and the fill-ups. However, the error should be pretty consistent -- off by about the same amount every time you check. There have been some reports that a sudden change for the worse -- the actual fuel economy gets quite a bit worse while the trip computer is still indicating about the same as before -- may be an indication of an injector gone bad. The trip computer calculates fuel usage based on what the injectors are supposed to be flowing, not what they actually are flowing, so when an injector jams open or starts leaking the fuel mileage goes to hell while the trip computer still thinks everything is fine.

If you suspect there is a problem with an injector, there are several steps to take. First, check the windings for shorts with a VOM. According to the Special Interest Car Parts catalog, the windings on an injector should measure 2.4 ohms at room temperature, and must not be shorted to the injector body.

Next, purchase one of the many fuel injector cleaners available that can be added to the gas tank.

If that doesn't fix the problems, the next step is to take the car to any place that specializes in fuel injection work. They can flush the injectors with a cleaner by just connecting a line to the fuel supply line and running the engine.

If this doesn't work, the next step is to depressurize the fuel system, disconnect the supply and return hoses, remove 24 nuts and remove the rack of injectors as a unit. Take it to the same fuel injection shop and have them flush it in a rig. With it separate from the engine, they can use much more powerful cleaners than they can on the engine. Best of all, even if their cleaning doesn't fix the problem, they will be able to tell you if any injectors are bad and which one; they can actually watch them spray on the bench.

If you're the adventurous type, you might try testing the injectors yourself. Unbolt the set from the engine and raise them up a few inches while leaving the fuel supply and return hoses connected. Jumper the relay in the trunk to allow operation of the fuel pump with the engine off. Immediately check that no injectors are dribbling fuel. Place a small glass jar around one injector. Connect a couple wires to this injector, and give it a quick zap with 12 volts. Note: do not leave 12 volts connected continuously; only give the injector instantaneous jolts. Observe the performance of the injector. Repeat for all injectors; you can empty the glass jar back into the tank between tests. Obviously, any injector that seems to perform differently than the others is suspect. No smoking is suggested during this procedure, and it would also be wise to avoid electrical sparks.

Tony Gardner reports that J. C. Whitney offers a device for doing such tests called an Injector Inspector. "This device is hooked up to the battery and one can select various combinations of pulses to operate individual injectors. They suggest using it with the injectors in-situ and timing pressure drops but I did not find this very convincing."

"Back lighting with a flashlight is helpful. You can also rig the device to facilitate pumping cleaner through each injector."

The injectors can be replaced individually, and there is nothing wrong with replacing only the one that's gone bad. Replacing the whole set is expensive, and usually not warranted.

The only differences between injector part number 0 280 150 163 (1985-91 front two), and 0 280 150 164 (1985-91 rear ten) is the length of the piece of hose attached to them. These injectors were fitted as a recall to cars with injector part number 0 280 150 105 (1980-85), so these injectors have no functional difference either.

The D-Jetronic cars had either 73143A/0 280 150 023 or 73143B/0 280 150 035 injectors. Some reports indicated that the last three digits indicate an orifice size difference, but Roger Bywater of AJ6 Engineering assures us that the number reflects a minor design change only and that all of these injectors have the same flow specifications. Note that the D-Jetronic injectors do not have the same flow rate as the Digital P injectors.

Richard Mansell quotes from a Jaguar publication on the changes for the 1992 models:

New smaller lighter fuel injectors allowing the fitment of a lower mounted fuel rail, giving improved control of fuelling at small pulse widths.

 Fuel injectors tend to be blamed for a lot of problems, probably more than they actually are responsible for. Michael Neal (Jaguar mechanic) says: "The only failed injectors I have seen have been from running water through them or running a car with a blown head gasket to death."


FUEL INJECTOR PULSE CHECKING: Robert Dingli suggests "an oscilloscope or a smart DVM with pulse width or duty cycle readouts can be used to check the injector electrical supply. Some modern ECU's can detect whether an injector is unplugged (or short circuited) and thus it's best to connect the electrical apparatus in parallel with the injector."


INJECTOR SEALS/NOISY FUEL INJECTORS: Each injector is mounted with two rubber rings, a small one at the tip and a large one around the body. These rubber parts isolate the injector from the manifold. There are two reasons you might want to replace your injector seals. The first is noise; Hunt Dabney says: "On the noise issue, I just replaced all of the injector seals and rubber mounts. My injectors are now really quiet!" The other reason, of course, is to eliminate vacuum leaks; the smaller ring at the tip is obviously critical for sealing the intake manifold, but the larger is also important for holding the injector securely against the smaller.

Frank Perrick points out that these seals are available at any auto parts store since they are standard Bosch items. The seals come in packages to service four injectors, so you will need three kits for the V12. Here's a tip for getting the guy behind the counter to get you the right parts: When he types all the make and model data into the computer, one of the screens he will face is where he must select the type part you're looking for. One choice is "fuel injector/carburetor gaskets", but if that selection is made it will come up with some obscure part number and claim it's not available. If, on the other hand, he selects "fuel injectors", it will come back with a listing for the "fuel injector seal kit". Or, Perrick suggests you can just carry an injector in there and say "I need these gaskets".

John Napoli found a different kit: "Standard Motor Parts SK-3 does the trick -- handles four injectors (eight rings -- 4 big and 4 little) for less than $4.00 US." A similar kit, the one carried by AutoZone, is GP Sorensen part number 800-9207. The kit offered by Discount Auto Parts is Tomco part number 27013.

However, not all kits are the same; some carry parts besides the two rubber rings. One such kit is Beck/Arnley # 158-0021 vp 1; B.J. Kroppe says, "The kit included the small, square-sectioned seal between the injector tip and plastic cup, as well as the large rubber washer which I believe isolates the clicking injector from the rest of the engine. The last bit that was in the Beck-Arnley kit was the tiny (nylon?) mesh filter that fits into the top of the injector fuel inlet. This is a neat design."

Kroppe also found a set from Python Injection, Incorporated, part number 200-120, with even more parts. "Description: large injector washer (isolator), square section o-ring seal, mesh injector inlet filter, injector tip cup (does not fit Jaguar injectors)."


IDLE STUMBLE: A stumble may be a sign of an intake manifold leak. Since the EFI senses manifold vacuum and meters fuel accordingly, it would make sense that an intake manifold leak would only cause a high idle, not a stumble or misfire. However, a manifold leak that is near the intake of one particular cylinder apparently has more effect on that cylinder than on others, and can cause an individual cylinder to run lean. This may be quite serious, since a lean-running cylinder is a candidate for a dropped valve seat -- a very expensive problem. If a leak is suspected, a can of WD-40 makes an excellent leak locator; simply spray on each area while the engine is running and see which spot makes a difference when sprayed on.

The gaskets between the intake manifold and the heads are common leak sources. It appears to be necessary to retighten the manifold-to-head nuts occasionally, especially shortly after a reassembly. In fact, just go out to the garage now and torque ëem all down.




RECOMMENDED READING: Roger Bywater wrote the definitive guide to the D-Jetronic system, published in Vol. 2, No. 2, Jaguar Quarterly (now Jaguar World), 1989. It is highly recommended that owners of cars with this system obtain a copy of this article.

For those with the D-Jetronic system and who have access to the World Wide Web, Ray Reynolds reports there is a site providing info on the system at It is maintained by Porsche drivers, but the EFI systems from that era were similar.


TRIGGER UNIT: The trigger unit originally fitted inside the distributor had magnetic reed switches and a three-wire connector that plugs into the EFI wiring harness. However, the replacement trigger unit uses Hall effect circuitry and has a fourth wire, separate from the connector; three-wire units are no longer available. The fourth wire must be connected to a 12V power source that is switched on with engine ignition. Unlike reed switches, the Hall effect sensors require a power supply.

Reportedly, the official Jaguar procedure is to connect this wire to the white wire at the ballast resistor. This, in fact, should provide the proper switched 12V source -- on paper, anyway. However, Ray Reynolds reports that the 12V available at the ballast resistor was so noisy that it caused the injectors in his car to fire wildly, resulting in flat spots and even stalling at some throttle positions. He simply reconnected this wire to a more reliable 12V source and his problems cleared up.

Jan Wikström points out that if you have the Hall effect unit, the testing procedure in the Jaguar repair manual won't work.


TRIGGER UNIT CONNECTORS: Peter French claims to be one of the few people that still has reed switches in his car rather than the Hall effect sensor. "I also had a good problem earlier with the 3-way connector to the injector switches. The centre (0v) pin had two wires going to one pin and they were "over-crimped". One wire had broken such that the reed switches had no common return unless the idle switch contact on the throttle switch was closed. Net effect was that the car would start and idle fine but cut out as soon as the throttle was opened!"


VACUUM SENSOR: Mike Morrin: "I would strongly recommend that all owners of cars with this system periodially check the manifold vacuum sensor for internal leaks. I stress this because when I needed a replacement for mine, I was told that new units are no longer available for the "early" calibration systems, and so I needed to find a good used one. This resulted in me driving around London (I was officially in the UK on business) for 2 days sucking on manifold vacuum sensors at all sorts of dodgy Jaguar parts shops. Of the dozen units I saw, only 5 did not leak, and only one of these was the correct calibration for my car. There are obviously a lot of cars on the road with leaky sensors.

"The sensor is the aluminium cylindrical thing right in the front right hand corner of the engine compartment. The test is simple, disconnect the center hose from the tee piece between the manifolds (above the distributor) and suck on it as hard as you can, there should be no hissing sound from the sensor unit. Then stick your tongue over the end of the pipe while releasing the vacuum in your mouth. The hose should stick to your tongue for several seconds before it (the hose) drops off.

"Internally the sensor has a barometer bellows, but there is an extra brass(?) diaphragm which provides the vacuum seal. This diapragm flexes every time the manifold vacuum changes and it is not surprising that metal fatigue sets in and causes a split around the outer edge. This is an axample of Bosch design which would make Lucas look good (if Lucas had not insisted on sticking their name over the Bosch logo!).

"If the sensor is leaking, then the engine will be running rich, particularly at part throttle, but this varies depending on the size of the split in the diapragm.

"I did attempt to repair my faulty manifold sensor, both by adding a rubber diaphragm and by soldering up the split in the brass diaphragm, but both of these repairs resulted in a significantly lean mixture."


D-JETRONIC REPLACEMENT: Roger Bywater of AJ6 Engineering says "it is not that bad a system and shouldn't be that troublesome really if it is properly set up (correct throttle balance is vital). Although odd bits like trigger boards and pressure sensors do fail occasionally, and injectors tend to jam if exposed to water contamination, the ECUs don't often go wrong (unlike early HE 6CUs) and really the wiring and connectors are the main problem areas as they age. It is a lot easier to go over the connectors, cutting back the wires and soldering them to the terminals, than to strip the lot out and install a new system.

"If the system is beyond economic repair and really does need to be replaced then I would have thought that by far the easiest route would be to simply fit all the hardware, including manifolds, plumbing and Constant Energy ignition, from any HE engined car from 1981-88 and use a 16CU controller (1986-91) and have us install the correct program for the early flat head engine (Super Enhanced option available). This setup has been used successfully on a number of V12 E types converted from carbs to EFI and has the great advantage that all the parts are familiar and easily available.

"Aftermarket ECUs sound great but in practice involve a lot of time and effort to get right and would be hard to justify for a standard engine. It would definitely involve rather more than "just wiring in a new processor" and any improvement in performance or fuel effcicency would be minimal.


Digital P


ECU INTERNAL CONNECTIONS: The harness connects to the ECU with a 35-pin connector. However, within the ECU itself, many of the wires from the harness are simply connected together. When trying to figure out how things work, it may be helpful to know what wires are connected internally. To save you the trouble of opening it up to see, I will provide the info here based on the ECU in my '83 (part number DAC 2597). The following groups of connectors are all wired together inside the ECU:

1-2-19 (ground)
8-9-27-28 ("B" bank injectors - open)
11-29 ("B" bank injectors - hold)
12-30 ("A" bank injectors - hold)
13-14-31-32 ("A" bank injectors - open)
16-17-34-35 (ground)

On the schematic in my repair manual, I have drawn boxes around each of these groups to remind me of these internal connections.


THROTTLE POTENTIOMETER: If your XJ-S has ragged throttle response as though it has a bad accelerator pump, here's some news for you: it doesn't have an accelerator pump. However, the throttle potentiometer, located underneath the throttle pulley on top of the engine, can cause similar symptoms when bad. When operating properly, the resistance across this pot varies smoothly as the pulley is rotated. If it is breaking up, it confuses the EFI computer on accels. The EFI computer, using inputs from the oxygen sensors, can usually keep the engine running reasonably well at constant throttle, but it stumbles during throttle changes.

The original Jaguar pot is both unreliable and expensive. Sounds like a Lucas part, but it's actually made by Bourns. Clearly, simply stamping "Lucas" on the top has an effect on reliability! You can visit Bourns at but there doesn't seem to be anything there specific to Jaguar.

If you wish to attempt to repair the pot, Matthias Fouquet-Lapar sends a procedure: "Carefully undrill 3 plastic melt points from the bottom. After about 2 mm you'll see a screw. Using a small screwdriver undo these screws.

"Open poti. It's actually a very good quality poti, (double contacts), but clearly showed excessive wear on light part throttle position. Using my VOM I could find several places where the carbon really had gone away.

"Mmmmh. I decided to cut off the 4 contacts, shorten them by about 1 mm and resolder them, so they would end up on the unworn surface. This is a pretty sensitive operation, be sure that you feel confident that you can handle it. You need some good small tools, a fine solder iron, some PCB cleaner and a very quiet hand.

"Resolder everything, adjust contact height as before to have the correct pressure when putting it back together. Also check if the contacts are really off the worn track.

"Put it back together, check again with a VOM. Smile, since you just saved a bunch of money."

"Total repair time was about 2 hrs. However, I think one can do this kind of repair only once, or maybe a single contact could be used instead of two, giving a potential of a third repair."

This author has had two throttle pot failures so far. In one of them, the conclusion was that the problem with this pot is that the wipers have little points on them so they contact the resistance track in a very narrow groove. Although the track itself is quite wide, the wipers cut little narrow grooves through it until they're riding on the ceramic below. I fixed it by replacing the wiper in entirety! I made a whole new wiper out of brass stock (available at any hobby shop) and fixed it to the plastic rotor with two tiny flush-head screws. The wiper configuration is completely different, with wiper arms cantilevered from one end rather than the double-ended wipers of the original, but most of the difference was simply to make it easier for me to make with hand tools. The

significant difference was that each contact touches the track with an area of metal about 1/8" wide rather than the point used before. Basically, the individual wipers were shaped by bending the end to form a folded edge. The pot was temporarily assembled with a piece of fine sandpaper laid over the track, and the rotor turned back and forth a few times to polish a smooth contact surface on the wiper.

Paul Bachman says, "this thread reminds me of a repair that I once did to a mechanical television tuner with a similiar failure mode. I went down to the local jeweler and bought some small scraps of 24k gold (about $2) and soldered them to the wipers. This provided a low resistance contact with very low wiper friction. Never had another problem."

In the author's other pot failure, the resistive coating that the wiper slides on was properly positioned to overlap the metal conductor at one end of the travel, but at the other end it barely touched the metal -- and was making intermittent contact as a result. Solution: go to an auto parts store and buy a kit for repairing a rear window defogger. This kit includes a remarkably tiny bottle of copper-colored paint, a tiny brush, and a template for repairing the line conductors on a rear windshield. Using the tiny brush, apply some of this paint on the metal conductor and overlap it just slightly onto the resistive surface. The result is a very reliable connection.

Another possibility is using a pot from another type car. Brian Sherwood relates his experience: "From my spare parts pile I found a TPS from a Ford, an '83 3.0L V-6, I think; part number E7DF-9B989-AA. It turned in the proper direction, same degree of travel (90 degrees), and had the same resistance as the original at both ends

of its travel (from .05 to 3.5K ohms). But no, it doesn't bolt right up. I cut a slot in a steel bushing to fit the drive lugs inside the Ford TPS. The other end of the bushing I crimped down to fit the D shape of the shaft on the bottom of the throttle pedestal, that the old TPS used to fit over. A piece of aluminum strap was used to clamp the Ford TPS to the bottom of the pedestal. The TPS fits snugly in the recess underneath, and loosening the strap allows adjustment. I cut the wire and plug from the old one, crimped on some female spade lugs to fit the terminals of the Ford unit (would have been neater to use the proper Ford plug, too, but I couldn't find that.) Red wire=wiper, green=high side of pot, yellow=low side of pot. With new TPS plugged in and ignition on, I adjusted the TPS to give .36 volts measured between red and yellow wires, at idle position. Bolted everything down, and went for a test drive -- worked great. Only difference from the original is that now it doesn't surge at cruise anymore!"

Peyton Gill: "I had hoped to be able to repair the pot by relocating the contacts on unworn surface. When the pot was opened I found that the contacts had worn through the conductive surface. One set of contacts had worn so long on the ceramic substrate as not to be usable... As worn as the old pot was I'm surprised that it didn't cause more problems than it did. The nature and extent of the wear would lead me to recommend that everyone check their pot with a VOM."

"What type Ford is it for? That's the same question the guy at Pep Boys asked! I told him it was for a V12 Jag (always enjoy the expressions I get from these counter guys).

"The pot I got at Pep Boys is a Borg Warner part # EC1046. This was matching cross reference from Brian Sherwood's Ford part #. The only challenge was machining a coupling to go between the throttle assembly which has a shaft with a flat and the pot which has a hollow shaft with two splines within. I took an old worn out 1/4 inch drive socket (5/16) and drilled out the drive end so I would at least have a little flat on one side for the throttle assembly shaft. The other end I took a file and worked a slot on each side to go up into the splined hollow shaft of the pot. Crude but effective.

"I reused the wiring and connector off the old pot. The replacement pot has three small male spade connectors which I had the luck of having the correct size female ends. About 1/2 size of standard spade connectors."

You'll like this tip: Roger Bywater of AJ6 Engineering reports, "Some years back when we needed a clean throttle signal we used a 5K wire wound pot on a bracket attached to the LH air filter ahead of the throttle and connected onto the end of the throttle spindle by a short length of 5/16 bore hose and clips. Might be a cheap solution to renewal of original."


THROTTLE POTENTIOMETER ADJUSTMENT: When you replace the throttle pot, the Jaguar manual says you must adjust it using their special electronic tester. Below is the alternate method.

The throttle pulley assembly must be unbolted from its tower to adjust the pot, meaning the linkages to the butterflies are disconnected; but the idle stop is part of the throttle pulley assembly and therefore is unaffected. You can even start the engine in this condition, as long as you don't go above idle.

Have the ignition on, and the throttle pot assembly rotated to the idle stop. The engine may be running or not. Connect a digital voltmeter (the old needle type just won't do -- go to Radio Shack and fork over the $40) to the red and yellow leads from the pot without disconnecting the pot from the harness. This can be done by pushing the probes under the insulation on the connector.

Adjust the pot by loosening the three small mounting screws and rotating the pot until the voltage reading is within 0.32 - 0.36V. Tighten the mounting screws and reassemble. Make sure the reading remains within limits when the linkage is all together and the engine is running at idle.


HESITATION: Mark Sampson had a bad battery in his car. "I was also experiencing some hesitation when warm, it has also gone away with the new battery. Interesting."


OXYGEN SENSORS: When the indicator on the dash tells you to, you probably should replace the oxygen sensors. They are critical to the correct operation of the EFI.

The oxygen sensors are available at local parts stores. There is no need to insist on a Jaguar part; a generic replacement sensor will work fine; Brian Sherwood suggests Bosch universal sensor p/n 11025 for the earlier non-heated sensor. The sensor for an XJ6 can also be adapted. Take an old sensor in with you (if possible) to make sure the new ones are the right thread, etc. Don't worry if the fluted sensor portion doesn't look exactly the same. If the wire is the wrong length or has the wrong connector on the end, use a crimp connector to connect the new sensor with the wire cut off the old sensor. Soldering is not recommended, as this wire may see temperatures high enough to melt the solder.

The dash indicator doesn't really tell you that the sensors are bad; merely that they've been in there a specified time, apparently 30,000 miles -- the "service interval counter" that turns on this light is connected to the speedometer sender, not the EFI system. There are reports of the sensors failing before the counter turns the indicator on. The counter is in the trunk, hidden under the carpeted panel on the left side near the filler cap. The back side of the black plastic box has an electrical connector, a metal device that looks like a solenoid, a single-wire connector, and a small protrusion pointing rearward with a tiny white plastic button on top. To reset the timer to zero when you replace the sensors, use a pointy object to push the center of this white plastic button in until you hear the thing reset.

MOTOR/AGE Magazine gives the following instructions for testing an oxygen sensor itself:

"An oxygen sensor can be tested safely with a digital voltmeter, but an analog (needle type) voltmeter will destroy the sensor in a heartbeat. A simple bench test has been suggested by Tomco Carburetors to check O2 sensor function. All you need is a vise, digital VOM and a propane torch.

"Lightly clamp the oxygen sensor in the vise with the sensor flutes facing upward. Attach the digital VOM leads to the sensor. If the sensor only has one lead, attach the VOM ground to the sensor body. Set the VOM on the 2V scale, then light the propane torch (using regular heating tip) and apply direct flame to the tip of the sensor flutes. The voltage should rise to about 0.8-0.9V within 60 seconds. Move the flame away quickly from the sensor tip and the voltage should drop drastically; move the flame back and forth and the voltage should respond quickly. If the changes aren't almost instantaneous, it's time for a new O2 sensor."

David Littlefield says, "AutoZone claims they will check your oxygen sensors on their equipment for free as part of their customer-friendly tool lending program. I think other parts places might do this as well, but I haven't tried it."

The later XJ-S comes with a "three-wire" oxygen sensor with a built-in heater to bring it up to operating temperature faster. These type sensors are more expensive than the earlier ones, but they still are available as generic Bosch parts; paying Jaguar dealer prices is not necessary. While it may be possible to use a cheaper non-heated sensor in a car that came with heated sensors, Rusty Beard says, "Do not use a non-heated sensor. It will increase the warm-up time. I just purchased some lower-cost sensors that are supposed to work fine, and are heated. The Part # is 341-90-06, and is described as a Oxygen Sensor, 3-Wire. Picked these up for $59.00 each. The Jaguar part is about $130 each."

Randy Wilson reports on replacing one-wire sensors with three-wire sensors in his 6-cyl sedans: "I did it to get the O2 sensor up to temp, period. I run tri-Y headers on my cars, which puts the tap point for the O2 sensor (the collector) a fair distance from the engine. It's enough distance that, combined with headers not retaining the heat in the exhaust stream, the sensors would cool off too much at around-town cruise speeds.

"Any of the Bosch three wire sensors from that era will work fine; BMW, Volvo, Jag, or universal. The black wire connects to the stock location, the two white wires go to power and ground. Doesn't matter which wire goes where. For power, I tap into the fuel pump relay so that the sensor is on when the pump is running. This is typical Bosch strategy.

"On the SIII, this will not get you into closed loop any quicker. The ECU will not go closed loop until the engine reaches a critical temp. But it will help in making sure the sensor signal during low load closed loop cruising is valid. Jaguar went to a three wire on the 87's for this reason."


OPEN-LOOP/CLOSED-LOOP OPERATION: On the US-spec Digital P system, the starter inhibit switch on the shift lever is also connected to the EFI system. In Park or Neutral (starter permitted), the EFI operates in an "open-loop" control mode, where the oxygen sensors are ignored and the EFI controls to a fixed fueling map. When in Drive or any other moving gear (starter inhibited), the EFI operates in "closed-loop" mode, adjusting the fuel mixture to obtain the correct oxygen sensor feedback.

For those of you who are subject to emissions testing, you should be aware of this. To put the EFI into closed-loop mode even in P or N, you must pull a shorting loop from a 2-connector socket near the ECU in the trunk.


CLOSED LOOP OPERATION: Michael Neal sends this info on checking the Digital P EFI system: "Monitoring O2 sensor feedback voltage just lets you know the EFI is in closed loop operation. This should be checked with a HIGH input impedance digital volt meter. A cheap meter will fry the system. Check the voltage at the O2 sensor connector with the sensor hooked up and the engine running. The voltage will fluctuate from 0 to 1 V. With a fairly fast updating meter you should get a voltage swing of at least .3 V. A .1 or .2 V swing shows a problem with the O2 sensor, open loop operation, or a problem with the sensor driver in the ECU.

"Verify the car is going into closed loop which is the .3 V or more swing. If it is then you need to check the injector duration. Most good meters have a duty cycle or millisecond pulse duration test feature to them. The pulse duration should be around 3 milliseconds."

Robert Dingli says: "It is pretty obvious when the system goes into closed loop control. While the open loop controller may be tuned to near lambda = 1 (relative air/fuel ratio where 1 equals stoichiometric mixture) the voltage output will ëflicker' around 0.5V or else sit at a low or high output. When under closed loop control, the perturbation due to the closed loop system can be clearly seen as a low frequency oscillation in the output."


FEEDBACK MONITOR SOCKET: Near the ECU within the trunk is a 4-connector socket on the end of a harness with a dummy plug in it. Two connectors are a black ground wire and a KB wire with +5V. The other two connectors provide a voltage between 0 and +5 that corresponds to the amount of correction the ECU is providing in its attempts to maintain the proper Lambda feedback. The GW wire (pin 4 on the ECU) is for the A bank and the GK wire (pin 10) is for the B bank.

While there is a special Lucas setting tool that plugs into this socket, Roger Bywater points out that an ordinary voltmeter can be used. 0 volts indicates the Lambda signal is high and the fuelling is weak; 5 volts indicates the Lambda signal is low and the fuelling is rich. 2.5 volts means the ECU is not correcting the fuelling at all, the engine is running right on the baseline map; if the ECU is in open-loop mode for any reason (cold engine, shifter in Park or Neutral, whatever) both connectors should be reading 2.5V.


IDLE MIXTURE ADJUSTMENT: The Electronic Control Unit for the EFI system is in the right side of the trunk, and there is an opening (sometimes covered with a rubber plug) in the side of the ECU. Within this opening is a shaft with one flat side, like those under radio knobs. This shaft turns in little snaps, and adjusts the baseline fuelling map (before Lambda correction) at idle.

If you've read the above few paragraphs, perhaps you have already figured out how to adjust it. Since you're probably in Park or Neutral, you will first need to pull the jumper loop from the 2-connector socket to put the EFI system into closed loop mode (with the engine fully warmed up, of course). Then monitor the voltages on the Feedback Monitor Socket described above, and adjust the idle mixture knob until the readings are as close to 2.5V as possible. This means that the baseline fuelling is adjusted as close as possible to optimum, and the feedback loop is not having to correct much. There is only one adjustment for both banks, so you have to do the best you can to balance between the two readings.

Roger Bywater elaborates: "The Lambda disable plug is for setting up only - the car runs in Lambda in "drive" but not in "park" or "neutral". I think it is because the early Lambda sensors would cool off and stop working. Later cars have heated sensors so run in Lambda continuously. The need for adjustment in Lambda mode is because if the ECU has to apply a lot of correction the Lambda swings make the idle unstable and it hunts up and down in time with feedback."

The real challenge, of course, is getting a tool in the hole to be able to turn the knob.


AIR TEMPERATURE SENSOR: The official XJ-S Repair Operation Manual, section 19.00.07, has a chart of air teminto closed loop control. While the open loop controller may be tuned to near lambda = 1 (relative air/fuel ratio where 1 equals stoichiometric mixture) the voltage output will ëflicker' around 0.5V or else sit at a low or high output. When under closed loop control, the perturbation due to the close/P>


WATER TEMPERATURE SENSOR: According to the Jaguar literature, the water temperature sensor provides a "minor" trim to the fuel schedules. However, when the sensor fails, the trim can easily become major. If the EFI thinks the coolant temperature is -60F (or some such ridiculous value), it's gonna be overly rich and have trouble starting.

Michael Neal sends this info:

XJ Late EFI Coolant Temperature Sensor Data Chart Created 06/24/94
Negative Temperature Coefficient Thermistor




Resistance (W)





































These things do suffer intermittent failures, with some Jaguar owners actually monitoring readings while driving to confirm that the problems were caused by this sensor. One of the connectors should give ~2.4 volts with the ignition on and the engine cold (the other one gives 0 volts) and gradually decrease to ~0.95 volts at normal operating temperature. Sudden changes in voltage mean the sensor is breaking up.

Neal: "Don't use Lucas brand. Get a Bosch part, cheaper and better. This chart applies to '79 and later fuel injected Jags. All the same." Good luck figuring out which Bosch part to get. The following part numbers have been suggested as being usable:

73198 -- original Jag part, 1979-89
DAC4737 -- Jag part, 1989-on -- apparently offered as replacement for 73198 on earlier cars
SNB 802 -- I dunno, it's supposed to be another number for the original part
0280 130 055 -- reportedly an exact replacement, but available only in Germany and very expensive
0280 130 026 -- apparently differs slightly in tolerance, but definitely works well
0280 130 023 -- apparently an earlier version

The sensor should cost about $30. If someone tries to charge you over $100, it's time to look elsewhere.

Note that the D-Jetronic system uses sensor 73142, and it appears this is not the same critter, not interchangeable with the later sensor.


VACUUM SENSOR (Digital P): The vacuum sensor for the Digital P system is within the ECU in the trunk, so a vacuum line is run the length of the car. On page 343 of the Haynes Jaguar XJ12 & XJS manual (#478) they state, "Commencing with... vehicle identification number XJS 107102, a vacuum reservoir has been located adjacent to the ECU to eliminate resonance from the inlet manifold vacuum sensor pipe." This reservoir is a plastic chamber about an inch in diameter and three inches long, fitted in the line just before it connects to the ECU. Would installation of a similar reservoir help earlier Digital P cars? Who knows?


ACCEL PROBLEMS: Vince Chrzanowski reports that his car wouldn't start accelerating until a couple seconds after he floored the accelerator, even if the tranny kicks down. Otherwise, the car seemed to run fine. "The vacuum line to the ECU in the trunk was partially blocked. A shot of compressed air cleared it out, and everything went back to normal."


RICH RUNNING: Richard Chapman sends this tip: "If you're getting rich running and fouling check to see if the pressure regulators mounted on both ends of the fuel rail are leaking fuel into the manifold via the vacuum line which connects between the reg and the manifold. With the engine idling pull off the hose at the reg, the engine speed will increase with the extra air entering the manifold through this pipe so block it with something. After running the engine for a minute or so you may notice the presence of some petrol coming from the regulator vacuum connection. If so replace it. They are not the same unit despite similar looks."


CABLES: Chrzanowski sends this report on a problem with his car: "It seems that the engine revs information is sent to the ECU from an amplifier unit atop the engine. The signal is sent via a thin shielded wire. Unfortunately, the wire is routed near some very hot areas and that heat melted the insulation between the center conductor and the shield, shorting the signal to ground. Replacing the defective section of wire restored the signal. In talking with some other local Jag owners, I've found that this is not an unusual problem."


COLD START PROBLEMS: William F. Trimble says, "My Haynes manual for the V12 (yeah, I know ... :-) suggests to remove the thermo-time switch altogether if you have cold-start problems."

Ironically, this author's Haynes manual (for the XJ12 & XJS) says "A problem can occur on 1980 models causing stalling immediately after cold start. This can be overcome by removing the cold start relay..."


IDLE STUMBLE: The mid-80's H.E. tends to "stumble" at idle when warm, even when it runs perfectly smoothly when cold or at higher throttle. This appears to be normal, although it is definitely disconcerting to an owner that spent the kind of money this car costs and expects better from his 12-cylinder engine. Fuel injector cleaning and/or replacement, spark plug replacement, and oxygen sensor replacement have no effect.

The only suggestion that seems to have any benefit comes from Chad Bolles, who suggests disconnecting and plugging the vacuum line to the right side fuel pressure regulator. This will cause the regulator to open up, and the left side regulator will open up to control rail pressure, so that the pressure is nearly unchanged but the flow circulating back to the tank increases significantly. The overall effect is minor, but it seems to reduce the stumble and improve the throttle response somewhat. Note: if the left side fuel pressure regulator is not connected directly to manifold vacuum, it may not be wise to disconnect the vacuum to the right fuel pressure regulator. One or the other of them must be connected to manifold vacuum, and on some cars a temperature sensor in the fuel rail is used to control vacuum to the left regulator to prevent problems with hot starting.

Roger Bywater responds: "The right hand regulator does not control pressure in the fuel rail and really only acts as a damping device to eliminate pump noise. I would not have thought that uncapping it would make any difference but a gauge on the fuel rail would soon show if there is any. In fact pressure regulation in the rail is governed solely by the left hand regulator."


ELECTRIC AUXILIARY AIR VALVE: In addition to the coolant-operated valve under the back end of the left intake manifold, there is an electric auxiliary air valve at the front end of the right side air filter housing. The official Jaguar repair manual -- at least the early versions -- don't mention this valve at all, and neither does the Haynes manual. So, a description from a later Jag manual is included here, thanks to Chad Bolles: "Each time the engine is started , the starter relay activates the 45 sec timer, which opens the supplementary air valve to provide increased bypass air flow for the first 45 sec of operation. Idle stablization: During air conditioning compressor operation, the supplementary air valve is opened via the idle relay to stabilize the idle speed. To prevent excessive idle speed with no engine load, the idle override relay is energized when Neutral or Park is selected. When energized, the idle override relay switches OFF the idle relay, thereby closing the supplementary air valve."

Victor Naumann adds: "Idle relay (black) and idle override relay (blue) are located on the left side of the radiator support in front of the A/C suction hose. 45 Sec timer is located on the right component panel, passenger footwell, left of fuse block." Those two relays aren't there on this author's '83, though.


INTAKE MANIFOLD SHORTCOMINGS: Bill White, an expert on the Helmholtz theory of intake manifold design, notes that the intake runners to the corner cylinders on the Jaguar V12 are longer than those serving the center eight cylinders. He performed single-cylinder rig tests on the different runners and found that, at the tuned frequency of the manifold, the corner cylinders produced 8% more torque than the other cylinders. This means that these corner cylinders are getting a larger charge of air than the others.

If this were a carburetted or throttle-body EFI engine, this wouldn't really make much difference. However, being a multi-port EFI, each cylinder gets the same amount of fuel. Since these corner cylinders are getting more air, they are running leaner than the other eight.

What can you do? Probably not much. If your system has oxygen sensors, they will sense oxygen from the leanest cylinders and control the fuel supply to maintain stochiometric there; the other cylinders will simply run a little richer than stochiometric, which shouldn't hurt anything except a little fuel economy and emissions.

Roger Bywater of AJ6 Engineering (and formerly an engineer with Jaguar) notes: "In this connection I would point out that sample cylinder heads were regularly checked in the flow room at Jaguar and it was not unusual to find a variation of 10% between the flow capacity of individual ports due to core shift on any one head... Also in about 1975 I remember carrying out some tests with calibrated injectors which showed a wide spread of exhaust CO emissions largely due to flow variations from cylinder to cylinder following no particular pattern. This was part of an investigation into why B cylinder bank was always a worse HC emitter than A bank. In fact it had nothing to do with airflow or mixture strength but was a result of the direction of piston rock at TDC relative to the spark plug position. These sort of things are not at all unusual on any production engine of the period."


PASSING EMISSIONS TESTS: If your car is too old and tired to pass a required emissions test, you should have it corrected. However, if you don't have the time or money to get this done right now, John W. Himes makes the following suggestion: "I add a gallon of denatured alcohol to the tank. This makes a very clean burning fuel. The car does not run as good with that in the tank, but it passes emissions very well."

Harry Trafford says, "I have used 99% isopropanol at 12% with some success. Brings down the CO%. Yes, the percentage is high and the idle had to be adjusted to keep running, but these were desperate people. I had them add 46 oz. to make 3 gal. of premium. After the test, they just bought more gas to dilute the alcohol and readjust the idle."

"I used 2-propanol because it's what was available at the time. I also had methanol, but it's corrosive and not a good thing to put in British cars."

Alcohol tends to be hard on some rubber products in the fuel systems of earlier cars, and British non-metallic parts have enough trouble without adding to their woes. Therefore, it is not suggested that you use this method more often than necessary.

Michael Minglin suggests another trick: "I managed to get the dealer's master mechanic aside for a private discussion. It seems that the V12 is a bear to get past emissions even on a good day. One trick they use is to rewire the vacuum lines so the diverter valve is pumping air into the exhaust manifold all the time. According to this mechanic this will bring the emissions down enough to pass."

Harry Trafford continues, "I bought a CO analyzer, too. It's a "Gunson's Gastester". This unit is not a professional system, and you need a lot of patience because it must clear to 0 after you make any adjustment. Says it reads in 1 minute, but I gave it more. It looks and feels cheap, but it worked for me by just following the directions. It's made in England."

"Here is the address for the guy that sells the "Gunson's Gastester" CO analyzer in Pennsylvania. He will answer your e-mail.

Ray at: r.d. enterprises, ltd.
290 Raub Road, Quakertown, PA 18951 USA
phone: +1 (215) 538-9323 fax: +1 (215) 538-0158


EARLY 3.6 AJ6 ENGINES: Roger Bywater of AJ6 Engineering reports that the 1983-87 3.6 AJ6 engine with Lucas 8CU EFI have a problem with worn engines. A worn engine draws less vacuum at idle, but since the 8CU bases fuelling on manifold pressure, it believes that the lower manifold vacuum means the engine is drawing more air and responds with more fuel -- making the engine run overrich. Since these cars lack Lambda sensors for trim it cannot correct itself, and the car runs worse and worse. Bywater says owners complain of "shunting" in trailing throttle, bad idle, and failing MOT tests due to excessive CO in the exhaust, even though the engine is not really all that tired and rebuilding doesn't appear justified. Of course, at full throttle the wear problems are insignificant and the engine runs fine.

Bywater suggests that disconnecting the vacuum advance from the throttle edge tapping and connecting it directly to intake manifold vacuum -- and readjusting the idle mixture and speed accordingly -- may help as a temporary measure if the engine isn't too bad. For a more permanent fix, AJ6 Engineering offers an ECU modification wherein a trimmer screw is added that can be used to correct for engine wear -- and can be corrected back in the event of an engine rebuild.


On to the Exhaust System


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