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.
D-Jetronic
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
www.estinc.com/porsche/djet.html. 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 http://www.bourns.com
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 -60ƒF (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
ƒC
|
ƒF
|
Resistance (W)
|
-10
|
14
|
9200
|
0
|
32
|
5900
|
10
|
50
|
3700
|
20
|
68
|
2500
|
30
|
86
|
1700
|
40
|
104
|
1180
|
50
|
122
|
840
|
60
|
140
|
600
|
70
|
158
|
435
|
80
|
176
|
325
|
90
|
194
|
250
|
100
|
212
|
190
|
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
e-mail: rdent@rdent.com
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
|