Jaguar XJ-S. Service manual — part 80

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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.”

IMPROVEMENTS IN LATER DESIGNS: Stefan Schulz reports: “I've just replaced pipes on the left side of my car's
exhaust system. Observations:

“Jaguar seem to have realized that some of the exhaust parts weren't brilliant and actually made some improvements, to
wit:

ƒ The intermediate pipe now has an extra kink so that it no longer prevents access to the gearbox sump bolts.

ƒ The locating pin arrangement for the over-axle pipe has changed so that it now looks a lot more reliable. Stress is

distributed over a much larger cross-section of the hanger-to-pipe joint now.

ƒ The old band clamps used in various places were labelled "Aeroquip". Presumably that points at use in flying

equipment. Which is a scary thought, as both which I removed used Nyloc nuts - not a smart thing to do on a hot
exhaust system! The new band clamps are actually different part numbers and sizes for different locations, and
they are designed like the mothers of all clamps. Extremely strong and wide - get the genuine Jaguar part for a
change. Well worth the money.”

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 magazine
road test and never see 100 mph again, 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

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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 a port into the exhaust tract, presumably because Jaguar used
the port in past models or anticipated the need for such a port at one time or another.

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 -- see page 45.

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.

Note that sometime in the 90’s, perhaps with the introduction of the 6.0 in 1994, the catalytic convertors were upgraded
with new designs that are far less restrictive. These new models reportedly do not have the honeycomb within the
downpipes themselves.

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°C, and remember this was at idle!!!

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“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.”

See page for warnings on how faults in the Marelli ignition system can burn your car to the ground and page 168
for how overheating cats can cook the Marelli crank sensor.

CATALYTIC CONVERTOR TEMPERATURE MONITORING: The regularity of the Marelli distributor rotor
failures (see page 160) 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. 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.” See page for
Summit.

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°F to 1600°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

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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, 300º 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.”

Note 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 equally. The typical cause of high cat temps is a misfire, and a misfire on
both banks can cause cat overheating on both banks. In fact, such a failure mode is described on page 167.

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 Jag Services.” See page 691.

AFTERMARKET CATS: LaRue Boyce says, “Caution on the aftermarket cats! Both pieces were not "true" and
couldn't be joined together without a lot of modifications.”

GENERIC CATS: Generic cats are available from J. C. Whitney for, like, fifty bucks each. Sure, you have to figure
out how to plumb them up, but considering the cost difference you have plenty of incentive.

CATALYTIC CONVERTOR GUTTING/REMOVAL/REPLACEMENT: Catalytic convertors are not inherently
particularly restrictive in an exhaust system; in a properly-designed system, removing the cats has very little effect on
performance.