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Exhaust System

  Experience in a Book
Exhaust System


MANIFOLD/DOWNPIPE NUTS: The nuts that hold the downpipes to the manifolds look like an extra long nut. They actually contain a self-locking helicoil, a special type of helicoil in which a couple of the coils near the center are deformed to provide some friction when turning. These helicoils are held within the nut by a staking on each end that deforms the thread just beyond the end of the helicoil. Whenever these nuts are removed and reused, it is recommended this staking be checked, and re-staked if necessary.

The helicoil design means the parent metal of the nut is thinner than normal nuts. This is countered by the nut being so long. If overtorqued, these nuts will split lengthwise, and internally grip the socket used to torque them. If this occurs, of course, the nut must be replaced. The stud is a normal fine thread, so a normal nut will fit. It is recommended that a stainless steel or brass nut be used due to corrosion problems at red-hot temperatures, and a locknut or lockwasher be used since a normal nut lacks the self-locking feature of the original. Obviously, a locknut that uses a nylon insert for self-locking is unacceptable.

Thomas Alberts says, "I have purchased the long nuts with the helicoil inserts from a Jag dealer, but since then I've seen them at Western Auto hanging on the pegboard in a bubble pack."

Some auto parts stores sell "stud nuts", nuts intended for use on exhaust manifold studs. Generally, they are longer than normal nuts and made of solid brass (no helicoil insert).

Another type of nut that looks right is the coupling nut sold in hardware stores for connecting lengths of threaded rod together. These are typically coarse thread, but even if you replace the stud these nuts would probably not work well. They are generally mild steel, so they aren't very strong and will corrode something fierce on an exhaust manifold.

Getting the exhaust manifold/downpipe nuts on and off seems imposing, but it usually can be done easily enough. The two outer nuts on each side can usually be loosened with a box end wrench and a lot of patience, and the two inner nuts can be loosened from underneath the car with a socket, universal, extensions and ratchet. Michael Minglin suggests, "For the outside studs I bent a box end wrench into kind of a "Z" shape so I could slip one end over the nut from the top of the car. I welded an old socket onto the other end so I could use another wrench for leverage, and use a torque wrench to put it back together."

An even better idea: find a 9/16" "distributor wrench". These wrenches are designed to tighten and loosen the bolt underneath a distributor on American cars, and therefore have a deep zig-zag to them.

Be sure to use anti-seize compound when assembling, whether using original nuts or substitutes.

Interestingly, Jaguar provides four threaded holes in each header when only two studs are used. If the threads in one pair of holes get boogered up, it should be possible to install studs in the alternate two holes, and rotate the loose collar on the downpipe 90 and reinstall.


TRANSMISSION/EXHAUST PIPE BRACKETS: When considering the mounting of exhaust pipes, it must always be kept in mind that the engine moves around on its mounts. When power is applied in low gear, the entire engine/transmission assembly can tilt significantly to the right. Typically, exhaust systems are rigidly mounted to the exhaust manifold and unsupported from there to the rear of the car, where they are supported by soft rubber mounts. The intention is that the pipes will flex enough over this length that the engine can move without damage or fatigue.

The XJ-S, of course, has a problematic transmission mount. Problems with the transmission mount can cause the engine to move more than intended, increasing the stress on the exhaust system.

The XJ-S also has several joints in the exhaust system around the catalytic converters, which happen to be in the middle of the unsupported span. These joints can vibrate loose under the stress. Add to this the fact that the catalytic converters themselves get very hot when running, and are made of a high-temperature, brittle material. There are reports of the catalytic converters actually cracking within about 50K miles.

Jaguar's solution is to add a pair of small brackets from the torque converter cover to the pipes. The intention is to render the portion of the exhaust system between the manifold and the catalytic converter rigidly attached to the engine/transmission assembly, thereby seeing no stress. The flexing of the system would have to occur from this point rearward.

This solution is mediocre at best. On earlier models, the torque converter cover is made of sheet metal; it is not strong enough for this load and generally ends up pretty mangled. The fix also causes noise; some of the vibration is transmitted to the sheet metal cover, adding a tinny sound to the car.

Later, a beefier cast cover was used. This may represent a solution to the problems, or at least part of one.


EXHAUST PIPE UNIONS: The three-bolt flanged joint in the exhaust pipe just forward of the rear axle is a very interesting device. The seal within it serves as a sort of ball joint. The angle of the pipes at the connection can be changed by tightening some of the flange bolts while loosening others. This enables corrections to alignment so the exhaust system doesn't bang against anything.

Unfortunately, few muffler shop weenies have ever seen such a feature. If they just zip the nuts on those bolts with the air wrench and proudly announce they finished the job in under 30 minutes, it's not likely things will line up.

If you have disassembled your exhaust system at this point and need to reassemble it properly, you will find it most helpful to have an assistant -- or a jackstand, if you are short of assistants -- hold the forward end of the rear muffler up in its proper position while you tighten the three bolts on this fitting. When you think you are done, grab the pipe and give it a good shaking. If it bangs anything, you're not done.


EXHAUST SYSTEM ASSEMBLY: Jan Wikström sends this procedure for assembling each side:

1. "Hang the bent pipe in place. Make sure the insulating compressed-fibre bead is in place inside the rubber. Smear Loctite exhaust joint compound in the joint and offer it up to the flange. Do the screws up looser than finger-tight.

2. "Insert the second muffler and engage its hanger. Smear the Loctite stuff on the sliding joint and join it up.

3. "Push in a finger-thick stick (conveniently brought by crazy dog) on each side of the tail muffler to jam it in the centre of its asbestos-lined pocket.

4. "Push the bent pipe clear of the brake calipers and jam in another stick between the pipe and the bleeding nipple.

5. "Shove the front muffler over and up until it sits right with another finger-thick stick between it and the heat shield. (memo: get a bone for the helpful dog)

6. "Look at the tail muffler to check that it isn't sitting too high or low through the see-saw action of the pipe. Waggle the pipe accordingly.

7. "While holding everything in place, tighten up the flange bolts about half tight.

8. "Fit clamp and slot cover piece, liberally smeared with you-know-what. And do up the sliding joint about half tight. Check that both hangers have ample space for the pipe to move.

9. "Remove all sticks, hold your breath and check clearances. If the exhaust can shake normally on the hangers without touching anything, tighten flange and clamp fully. If not, first apply brute force as required...

"The Loctite stuff is great; when the heat comes on, it foams to maybe three times its volume and makes a tight seal. A tap with a hammer will break it when you want to open the joint."


HEAT SHIELDS: The Jag seems to have them everywhere: those hokey little pieces of sheet metal that seem to do nothing but get in the mechanic's way. Not so! While some performance cars are designed to go fast for the duration of the magazine's road test and to never see 100 mph otherwise, the Jaguar is designed for the Autobahn -- not just 140 mph, but 140 mph all day. Under these conditions, the exhaust manifolds and downpipes can get red hot. Items that hot radiate heat (emit heat as infrared light - you can feel it from a distance). The belts, electrical insulators, boots, hoses, O-rings, etc. (all of which happen to be black) absorb this radiant heat and cook. The heat shields are not there for passenger protection or to aggravate the mechanic; they are necessary to prevent the rubber and plastic parts from destruction.

Particular attention should be paid to the heat shields around the catalytic converters, because they get hot even when you're driving slow. If these heat shields are not in place, the boots on the steering rack ($$$!!!) won't last long.

Heat shields are no more complicated than they look. You can easily make them out of scrap sheet metal. Hoses and the like can actually be provided considerable protection by merely wrapping them with ordinary aluminum foil.


HEAT SHIELD BOLT SEALS: The heat shields over the exhaust manifolds on the H.E. are held on by two bolts of different sizes. The larger rear bolt is actually a plug for an unused EGR port, presumably the same port that is actually used for EGR on the emission-controlled pre-H.E. engines. Not eliminating these ports in later models was probably a pretty wise move on Jaguar's part, since emission regulations have been notoriously difficult to forecast, and it also meant not having to change a part number or stock different manifolds for different cars.

This fat, stubby bolt is sealed with a copper washer. If your local auto parts store has a rack of red cards titled "Help!", it probably has a package of two sealing washers that are the correct size: number 66265, "Brake Hose Bolt Washers", ID = 33/64", OD = 45/64". These same seals fit the banjo bolt under the oil pressure sender.


CATALYTIC CONVERTORS: According to Randy Wilson, the XJ-S has "two different types of catalysts in series. The first cat after the engine is a three-way. The second is a single function reduction cat." Since it is a dual exhaust system, there are two of each for a total of four catalytic converters.

There is a honeycomb insert in the downpipe, immediately adjacent to the exhaust manifold. This is part of the catalytic converter system.

According to John G. Napoli: "I happened to be at my local Jag dealer as someone was taking delivery of a new pair of V12 head pipes. I noticed that these pipes did not have the corrugated metal inserts in each of the four down tubes. These inserts are ostensibly part of the catalyst system, and are present on many V12s (including my '82 XJ-S H.E.). Anybody who has seen head pipes with these inserts in place would reasonably conclude that they add a lot of restriction to the exhaust. The replacement pipes I saw today have a different shape to the cats -- more streamlined and seemingly smaller than the cylindrical cats on the head pipes with the inserts. The parts man said that both types of pipes are available (with the inserts and without), both are fully certified smog-wise, and that the design without the inserts is considered a better performer because of the reduced back pressure. The downstream cats are required with both designs. I did not inquire as to price, but it seems that anyone replacing their cats should consider these replacements, as the Jag V12 rewards you for anything that helps it breathe better."


CATALYTIC CONVERTORS -- MELTDOWN/FIRE: When a cylinder fails to fire, the unburned charge of fuel and air is pumped into the exhaust system. If the catalytic convertors are up to temperature and operating, they will "burn" this mixture, and get hot as a result. If there's a lot of misfiring going on, there is a risk of a serious fire starting at the cats and possibly destroying the entire car. Roger Bywater expounds at length on this concern: "Back in the 1970's when I was working in Emission Control Dept. at Jaguar one of my responsibilities was complying with Japanese Heat Damage Tests. Amongst other things this meant having a catalyst overheat warning system (via a thermocouple in the cat) and the test procedure called for one spark plug to be disabled while idling to prove the system worked. The mixture from the dead cylinder would then be burnt in the catalyst which would obviously get a bit hot. On the old oxidising catalyst systems this was not unduly dramatic, but even so because the carb XJ6's had the cat well back under the car, if driven any distance in this condition, they could end up with the rear seat springs popping through the top of the seat!

"Now when we started using Lambda sensors on the 4.2 EFI engine the situation changed alarmingly. When a plug was disabled the Lambda sensor would detect the spare oxygen from the dead cylinder and the system would react as if the fuelling was too weak so swung to the rich limit in trying to correct it. The catalyst now would be getting a supply of air and extra fuel and would start to glow in no time at all, even at idle.

"Of course the main reason the cat got hotter on the 4.2 EFI with Lambda was not so much because of having feedback as because it was a lot nearer the engine than had been the case with the oxidising cats. Also one cylinder out on a 4.2 6 cyl puts through a lot more fuel than one out on a 5.3 12 cyl so the 4.2 cat had a lot more to burn.

"In fact on the carb engine with oxidising cat we had to disable 2 cylinders to provoke the cat to overheat enough for the test. On the EFI with a 3 way cat one cylinder was more than enough and we had to keep reconnecting the lead periodically during the test to stop the cat rear cone temp going over 1000 degrees C, and remember this was at idle!!!

"I am sure this is far from unique to Jaguars and is made worse by the presence of a pressurised fuel supply with the potential to make sure almost any underbonnet fire will have catastrophic consequences.

"Really if any catalyst car develops a misfire it should not be driven. It is a bit like when the oil warning light comes on - driving another couple of miles could prove to be very costly! It is perhaps realisation of this sort of problem that prompted California ARB to come up with OBD & OBD2 with the requirement for really powerful fault monitoring techniques."

Read the warnings on how faults in the Marelli ignition system can burn your car to the ground and how overheating cats can cook the Marelli crank sensor.


CATALYTIC CONVERTOR TEMPERATURE MONITORING: The regularity of the Marelli distributor rotor failures has resulted in suggestions for monitoring the temperature of the catalytic convertors in hopes of avoiding expensive damage. This plan has a distinct advantage over monitoring the various possible causes of overheating, since it will alert the driver to overheating problems in the cats irregardless of whether Marelli is at fault or something else. A jammed fuel injector, a faulty fuel regulator, ECU problems, any number of things can cause a catalytic convertor to run hot, and with a large engine such as the Jag V12 it doesn't take too large a problem to cause some serious overheating.

There are lots of ways to monitor cat temperature. Some sort of fusible link -- perhaps even homemade, like out of silver solder or something -- could be installed on or near the cats that would melt and break a circuit when the link's melting temperature was reached. Some sort of bimetal strip or coil could be used to open and close a contact at a certain temperature. A thermocouple or two could be installed in, on, or near the cats. If you really want to get tricky, there have been mercury thermometers made that will close contacts via the mercury itself when it reaches a particular temperature. There are optical sensors that will detect and measure infrared radiation. For a really half-assed indicator, a pair of normal insulated wires could be twisted together and installed so that a hot cat would melt the insulation and cause a short.

The first problem is figuring out what temperatures we're talking about. Bob Gallivan forwards a guideline: "This is from "How to Tune & Modify BOSCH Fuel Injection" by Ben Watson: The minimum operating, or light off, temperature of the converter is 600 F, with an optimum operating temperature of about 1,200-1,400 F. At a temperature of approximately 1,800 F the substrate will begin to melt."

In Japan and the Middle East, catalytic convertor temperature monitoring systems are required by law -- so you may be able to obtain the necessary parts from Jaguar. Richard Mansell says, "Browsing through the '87 XJ-S parts manual I have found the bits and pieces used for the catalyst monitoring on the Japanese spec cars. They appear to use a catalyst mounted thermocouple, DAC1226, along with a little black box, DAC6943, known as "Module-catalyst switching". Also listed on the same page are sensor-thermal, DAC1043, which appears to be bolted to the floor although it does not say where. There is a harness, DAC3573, to plug it all together."

Andrew Corkan and others have pointed out that you can "buy a commercial dual-needle exhaust temperature monitor from Summit racing. About $250 US, works but has a big goofy dial you will have to mount."

Michael Aiken suggests, "If people are serious about monitoring cat temperature there are relatively inexpensive probes and gauges - they are used on snowmobiles to monitor exhaust gas temp for tuning. They even have digital gauges. They can be found at any snowmobile (motorcycle) shop or snowmobile catalog.

"I looked into a setup in a catalog I have. A dual analog Westach gauge (one 2" gauge, two needles) goes for about $85, a 3" gauge is $100. Two probes ($30 ea) would be $60 for a total of $145 to $160. The two needles in the gauge point at each other and should register the same under normal operating conditions. One rising significantly above the other would indicate a problem! The probe is mounted in a 3/16" hole with a stainless steel clamp (no welding). The gauges read from 400 to 1600 degrees F. The only problem I see is the leads to the gauge from the probe are only 4 feet long. It's not far from the exhaust in a snowmobile to the gauge panel."

Corkan again: "A DIY thermocouple option accessible to everyone might be to get K type thermocouples (~$20 each). Then get a specialized thermocouple amplifier (Linear Tech #LT1025 is a cheap option, ~ $10 each) and wire the output of the amplifier to a comparator (a cheap one from Radio Shack) that will turn on a light when the amplifier output goes above a certain point."

Since there may be some question about just how hot is too hot, John Arthur suggests, "What we need is a reference temperature. Fortunately the excellent design of the car means that one has already been provided at enormous expense. That's right -- the other cat! What is needed is a measurement of the difference of temperature between the 2 cats. There are industry standard thermocouples that are used in labs and workshops for measuring oven temperatures. Some are simple and could be clamped to the outside of the cat and others have a threaded boss that would be best screwed through the exhaust just behind the cat. Connect 2 of these thermocouples, one per cat, back to back and the voltage developed across the ends will be proportional to the difference between the temperatures of the 2 cats. You have to take the thermocouple wires back to the electronics but they are available with a length of 2 metres which should be enough. These things are reasonably linear and a temperature difference of, say, 300 degrees Celsius would give an output of around 12 millivolts. Some enterprising electronics guy could doubtless produce a simple amplifier that would enable a warning light or buzzer to sound. Two amplifiers and 2 gauges and you could read the actual temperatures. There are even ICs available which compensate for the slight non-linearities of the thermocouple.

"I have found a reference to Analog Devices AD595 as a thermocouple amplifier for Type K thermocouples. However these cost over 12GBP plus tax in 1996. A simple generic op. amp. such as a 741 or equivalent would cost pence/cents and do the same job of alerting you to a major temperature difference."

Matthias Fouquet-Lapar notes that monitoring only the difference in temperature may not be a good idea, since failure modes other than the Marelli rotor failures may threaten both cats. "The absolute temperature is important, the concern is destroying a cat due to high temperature. If you are for some reason running very rich on both cats, temperature will be high on both and the fault will go undetected." In fact, such a failure mode is described.


CATALYTIC CONVERTOR NOISES: Gerald Foster reports, "The dealer is turning out to be not so dumb. If I had gone with them I would have saved buying a power steering pump and water pump the independent dealer sold me. (Yes, a bad cat can sound like a grinding power steering pump)."


CATALYTIC CONVERTORS -- CHECKING: Greg Maddison suggests that you can visually check the front cats for plugging by disconnecting the pipes between the first and second cat, unscrew the oxygen sensors, and insert a small light into the hole. "I used a small Mag light with the shade removed." Looking into the back end of the cat, you should be able to see the light through the core.


CATALYTIC CONVERTOR REBUILDING: Greg Maddison says, "The original Jag parts are $800 each so you can see how changing all four would be quite an investment. I found a company that rebuilds them for much less than new ones cost, they are called Jaguar Services."


AFTERMARKET CATS: LaRue Boyce says, "Caution on the after market cats! Both pieces were not "true" and couldn't be joined together with out a lot of modifications."


EXHAUST PIPE TIPS: The XJ series Jaguars have a unique style of tip on the exhaust system, a sort of S-shaped extension that places the outlets right out on the corners of the car. This is not a mere styling feature; these tips were supposedly designed to correct a problem with exhaust fumes recirculating back into the car. The shape puts the outlets out into the airstream coming around the car rather than into the dead air space behind the car. If you wish to replace these extensions with some generic replacement tips, be prepared for fume problems when driving.

Of course, the aerodynamics are significantly different between the various XJ models and all of them have the S-shaped tips. While the fume problem may be the case with some of them, it's not likely to be the case with all of them. Some people claim to have no problems, while others complain bitterly of the odors. Pay your own money, take your own chances.

You might not really want to replace them anyway. The generic pipe tips available locally are usually cheap chrome-plated steel, and the chrome comes off quickly and the steel underneath rusts away to nothing. The stock Jaguar tips are made of stainless steel, so there is no plating to flake off. No matter how dirty they get, they can always be made to look new again with a little work with some Comet cleanser.

J. C. Whitney offers stainless steel exhaust pipe tips, basically a 9" long straight piece of stainless steel pipe cut off either straight or at an angle at the end and held in place with two setscrews. The ones to fit "1-5/8" to 1-7/8" OD pipe" will fit the stock XJ-S muffler nicely; catalog number 12xx0949U for the straight cut end, 12xx0952Y for the angle cut tip. Note that the outlets on the mufflers are turned down slightly, so these tips will not sit horizontally but will angle downward a bit.



On to the Drivetrain


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