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 90ƒ 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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