Jaguar XJ-S. Manual - part 68

 
 

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“In fact, the over-run valves were notorious for losing spring pressure and causing a high or erratic idle speed.  The 
practical solution was to just tighten the nut about 2 turns.”  Basically, adjust it just tight enough that the valves are 
closed at idle -- which can be confirmed by putting a finger over the hole in the filter housing with the engine idling.  
“Setting the over-run valves by checking with finger over the hole is as good as any method.  As long as they don’t leak 
at idle but work when you blip the throttle and release it to create an over-run condition they will be OK.  You will 
probably hear them anyway.  Again from memory I think they were meant to start to open at 20" Hg but by their nature 
they are more a sort of progressive leak rather than a step change device so manifold vacuum could exceed this value to 
a small extent.” 

 

ACCELERATOR PEDAL -- EARLY MODELS:  Mike Morrin says, “The Jaguar pre-HE service manual seems not to 
have been updated from 1975 to 1981, which is fine for my car, but I have seen a few things which are obviously 
wrong for most of the pre-HE cars.  A minor example of this is the RHD accelerator pedal assembly:  The only type 
shown in the manual was only fitted to the first 200 cars.” 

 

THROTTLE CABLE -- ELECTRICAL PROBLEMS:  Ron Brock says, “The starter was acting as though I had a low 
battery. After replacing the battery to no avail, I started in on the starter. The car would crank over fine sometimes and 
other times it would barely turn. After receiving several "tips" about checking the electrical connections, I went thru 
them from the battery forward. 

“My problem became clear one night in the garage, I was attempting to start the car with the bonnet up, and it was just 
dragging.  It was then that I noticed a whiff of smoke coming off the back of the engine.  I walked around to the front of 
the car to determine what was "smoking," it was the throttle cable, and was it hot (by the way this also solved my 
sticking throttle problem).  It seems that the engine was grounding itself thru the throttle cable. I was unable to remove 
and clean the standard grounding strap at the engine, so I added a second strap that I purchased from NAPA for $4 and 
the car has been fine.  Absolutely amazing.  I would seriously recommend that anyone have starter/charger problems 
explore this cheap and easy fix first.”  See page 553. 

 

THROTTLE LINKAGE BUSHINGS:  There is a throttle shaft on each side of the engine, parallel to the heads, that 
transmits throttle motion to the butterflies.  The rear end of this throttle shaft, along with some linkage, is supported by 
a plate bolted to the rear of the intake manifold.  There is a rubber bushing in the plate for the shaft to turn in.  This 
bushing is probably shot -- British nonmetallic parts again.  This bushing will dry up, crack to pieces, and fall out, 
leaving the throttle shaft to wallow around in the opening.  In fact, this is another example of a 100% failure mode in 
the XJ-S; if you haven’t already replaced the bushings, they are probably bad right now.  The effect on the throttle 
operation is not good, as it tends to screw up the sync of the two butterflies with each other and with the throttle pot in 
the turntable.  There are also reports that this problem can have adverse effects on emissions tests, especially if one 
bushing is gone and the other is still on the job. 

The part number for the original Jaguar rubber bushing is C34388.  However, I don’t think anyone in their right mind 
would recommend the original bushing -- it’s rubber, for heaven’s sake.  Rubber throttle linkage bushings are a relic 
from cars with rigid linkages from the pedal to the carbs; since the engine moved around on its mounts, some means 
had to be provided to connect the pedal on the car to the butterflies on the engine without the engine’s motion affecting 
the throttle position.  The solution was to have one rotating shaft with one end mounted on the car and the other 
mounted on the engine; the pedal was connected to a lever right next to the mount on the car, and the butterflies were 
actuated by a lever next to the mount on the engine.  The pedal twisted the shaft which opened the butterflies, and 
engine motion would rock the shaft back and forth without twisting it.  Rubber bushings were needed at the ends of 
such shafts to isolate engine vibrations from the rest of the car, and to allow the small angular motions of the shaft 
without any binding. 

None of this applies to the XJ-S.  The motion of the engine relative to the car is dealt with via a throttle cable.  The 
linkages where the bushings are used involve shafts that are mounted at both ends from the same intake manifold, so 
there is no relative motion.  Totally rigid bearings will work fine. 

 
 

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This application really begs for nylon bushings, but bronze bushings would probably work just as well if you can install 
them so they wouldn’t rattle or jingle.  The shaft is 5/16”, and the hole in the plate is 1/2”.  With a little looking, it 
should be possible to find a suitable generic bushing to use here.  A bushing with a single lip will work; retention won’t 
be a problem since it is effectively trapped. 

Replacing this bushing looks difficult; it appears to require removing either the throttle body or the linkage support 
plate, either of which is a pain.  You may choose to remove the linkage support plate, since it will permit you to fiddle 
with the bushing installation away from the car -- or take it with you when shopping for a bushing.  If you go this route, 
make sure to have two EAC2650 gaskets on hand for each side; see page 41. 

David Littlefield describes a shortcut:  “I was able to replace both my bushings today in my '88 XJ-S without removing 
either the throttle body or the linkage support plate.  I first loosened the pinch bolt on the short rod that is between the 
bushing and the throttle body.  I then removed the spring clip that holds the spacer that fits against the bushing.  By 
sliding both the spacer and the pinch bolt as far as I could towards the front of the car, I was able to gain enough 
clearance to push the rod towards the rear of the car for the ball end of the rod to come out of the throttle body fitting. 

“Once I had the rod out of the car, I was able to move the pinch bolt even further back-- past the knurled portion and 
right against the shoulder.  I then took the new bushing to my bench grinder.  I carefully ground down almost all of the 
lip on the beveled side.  I put the bushing on the rod, then fitted the rod back in the car, pushing the rearmost end in first 
and then putting the ball end in the throttle body fitting.  I then pushed the bushing into place, while still on the rod.  
Removing almost all of the beveled lip on the bushing allows the bushing to be pressed into place while on the rod, 
since distortion of the inner diameter of the bushing is minimal.  Replacing the spring clip behind the spacer and 
resetting the pinch bolt completes the procedure. 

“As stated in the book, a bushing with a single lip will work since the bushing is effectively trapped.  There are no real 
worries about the bushing slipping back on the rod because the spacer and spring washer hold it in place. 

“I used the factory bushing for this procedure, but I see no reason why it wouldn't work with others.”  Littlefield gets 
two demerits for replacing a British nonmetallic part that failed with another part just like it -- but at least, using his 
method, it won’t be too difficult to replace them again. 

Jeff Elmore offers a different shortcut:  “I remembered someone saying that it was easy if you did (something) and slid 
the shaft back through the throttle housing.  Well, after some contemplation, I figured he meant removing the screws 
from the shaft-to-plate and sliding the plate out of the shaft and sliding the shaft forward through the housing.  Well, it 
worked like a charm and the bushings were replaced in about 20 minutes.” 

If you’d like to try a more expedient fix, John Napoli describes a method he credits to Gerry Duff:  “We just cut a 
couple of pieces of rubber hose.  Slipped right onto the shafts, and is the right OD.  No tools or disassembly needed.  
Been working fine for almost a year, and when they wear out 5 minutes to do it again.  When the repair was done, the 
rubber hose sections were left long enough so that they could be turned around if excessive - and quick - wear was 
realized until a more conventional repair could be effected.  Last week I asked the owner how it was holding up.  He 
pulled the hoses, and stated that there was no visible sign of wear at all -- he had never needed to turn 'em around.”  
Note: this author tried using this method on a friend’s car, and couldn’t get it to work.  It apparently requires a 
particular type of hose we didn’t have on hand; we were trying to use fuel hose, which was a little too fat to fit through 
the hole properly and tended to “walk” off the shaft when the throttle was moved repeatedly.  Napoli suggests that the 
hose used with success may have been vacuum hose. 

As Leslie Winfield discovered, a generic bushing with no lip will work if you can figure out how to hold it in place -- 
and can make installation almost as easy as Duff’s fix.  “I purchased a 1/2 inch x 5/16 inch bronze bushing 1 inch long, 
and two 1/2 inch outside snap-rings.  I ran a 5/16 bolt through the bushing, pinched it with a nut & chucked it in my 
drill press (poor man's lathe).  I shaved about .005 inch off the OD with a flat file, used a hacksaw with a 24 tpi blade to 
cut a groove about 3/32 of an inch from each end deep enough for the snap-ring, and then cut the bushing into two 
halves.  With a little fiddling, the snap-ring can be threaded to the inside of the support plate, the bushing can be slid 
over the shaft and through the hole in the support plate (grooved end first), and the snap-ring positioned into the groove 
on the bushing.  The bushing is now captured on the shaft, and almost no play is observed with this setup.  It doesn't 
look too bad, cost $1.07, and it should last as long as my ’79 XJ-S does.”  Of course, if you happen to have purchased 
bushings with a flange, you can still use this method by cutting the groove in the opposite end from the flange.  While 

 
 

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buying these bushings, you might want to buy a spare one in case you ever need to replace the thingy in your Lucas 
distributor -- see page 142. 

Home Depot offers a nylon part that can be installed exactly the same way that Winfield installed his bronze bushings.  
It’s stock number 30699 87298, described as “1” x 1/2” SPACER - NYLON”.  All of Home Depot’s “nylon spacers” 
are marked for their OD and length, with no marking on the package at all about their ID; there are several ID’s 
available, including 1/4”, 5/16”, and 3/8”, but other than the stock number they are all marked the same.  If they happen 
to be out of the 5/16” ID bushings, you could purchase the 1/4” ID bushings and carefully drill them out. 

After bushing replacement, the linkage should definitely be adjusted as described below. 

 

THROTTLE LINKAGE ADJUSTMENT:  If the butterfly stop screws have been disturbed, the linkage bushings have 
been replaced, or any other tinkering has been done that could mess up the linkage adjustment, it should be readjusted.  
Also note that several owners have found that a rough idle cleared up when this procedure was undertaken, even 
though one wouldn’t expect it to make any difference; the crossover pipe should deal with any imbalance between 
sides. 

I’d like to simply dump the description of this procedure off on the repair manuals since any decent repair manual 
would provide a step-by-step procedure that’s easy to follow, but unfortunately the procedures described in the ROM 
(Section 19.20.11 followed by 19.20.05 -- not 19.20.25, as is indicated in Step 10 even though such a section doesn’t 
exist) is only barely discernible and the one in the Haynes manual (Chapter 3, Section 47 followed by Section 37) is 
bloody awful.  I will endeavor to explain the process here more clearly. 

In further developments, the mid-90’s cars with OBD II required much more careful adjustment to avoid fault 
warnings.  TSB #19-36, dated 10/95, says:  “Diagnostic Trouble Codes (DTCs) may be set if both throttle valves are 
not synchronized to begin opening at the same time. The Manifold Absolute Pressure (MAP) sensors will detect a 
difference in pressure between the two intake manifolds, which leads to the setting of DTCs. 

“The most common codes found are: P1106 P0106.  The following DTCs may also be set:  P0107, P0108, P0125, 
P1107, P1128.  Poor throttle valve synchronization may also cause customer complaints of poor driveablity and idle 
quality.” 

TSB #19-36 provides improved procedures for adjusting the throttle linkage on the OBD II cars, but it can be applied to 
all V12’s with EFI.  As a result, these improvements have been incorporated into the following descriptions of the 
adjustment procedures.  Joe Bialy and Richard Mansell provided some interpretations of this TSB.  Mansell says, “I 
suspect there are different TSB's for the different models.  I guess the contents are the same but for a start the throttle 
pedestal is different on my '96 XJ12 compare to the XJ-S I owned so the picture with the TSB is different.” 

The linkage requires five distinct adjustments, which must be done in order because each affects the others.  Proceed as 
follows: 

Before starting, remove the air filter covers and elements.  It is highly recommended that you clean the throats before 
proceeding, using some sort of solvent (see THROTTLE STICKING below) to get the areas where the edges of the 
butterflies contact the housings nice and clean.  Rather obviously, check the condition of such things as the linkage 
bushings (see above) before doing the adjustment. 

Disconnect the crossrods from the throttle pulley by prying them off the ball joints. 

Loosen the locknuts and turn in the butterfly stop screws until they don’t interfere with the butterfly motion.  Make sure 
the stop screws turn easily enough to turn with your fingers; if they’re hard to turn, you may need to take them out and 
clean up the threads. 

Loosen the clamp on the lever at the rear end of each butterfly shaft, directly below the crossrod attachment.  This lever 
is clamped onto a splined section of the butterfly shaft, and since the clamp takes on some toothiness the first time it’s 
tightened it tends to move in snaps.  While holding the butterfly closed move the lever at least one snap in the open 
direction, both to ensure you have the clamp loose and to make sure the stop within the linkage here isn’t interfering 
with the butterflies closing. 

 
 

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Adjustment 1:  Loosen the screws holding each butterfly disk in its shaft, gently press the butterfly closed so that it 
makes secure contact all the way around, then retighten the screws.  This centers the disk in the opening. 

Adjustment 2:  Set the butterfly stop screws.  This screw prevents the butterfly from quite fully closing; if it closed 
completely, it could easily jam, especially with temperature changes such as shutting off a hot engine and then trying to 
start it the following morning. 

To adjust the stop screw requires a 0.002” feeler gauge.  The earliest Jaguar repair procedures specified a 0.004” (0.105 
mm) gauge, but after 1978 it was changed to a 0.002” (0.05 mm) gauge -- which works fine for the earlier cars. 

Insert the feeler gauge at the bottom, push the butterfly firmly closed on it, and give the feeler a little tug so the butterfly 
grips on it.  Note that the butterfly should grip the feeler securely; if everything is right, it’d take a firm pull indeed to 
get it out.  If it seems to slide out with some smooth resistance, it’s probably not gripping it correctly but rather just 
getting close enough to be bending the feeler around the curvature of the throat.  Make very sure there are no throttle 
stops keeping the butterfly from closing fully on the feeler. 

While pulling gently on the feeler, turn the stop screw with your fingers just until the butterfly starts to let go of it.  Try 
it a couple of times until you get the feel of the process, at least well enough that you can do both sides the same way. 

Without moving the stop screw from this position, take your fingers off of it and put an open-end wrench on it.  Tighten 
down the locknut while holding the stop screw still with the wrench. 

From the TSB:  “An improved factory procedure for setting the throttle stop screws, using an air flow measuring gauge, 
was introduced during the 1994 MY production of V12 engines.  All 1995 MY V12 engines are set at the factory using 
this method.”  Many garages actually have this air flow measuring device since it is indispensible for syncronizing SU 
and Zenith Stromberg carburetors.  It should work fine here, although you might need to remove the inner half of the 
air filter housing to be able to get it to sit on the intake properly -- and then you’ll need to plug the bolt holes, since they 
create a vacuum leak.  If going this route, you should set one butterfly stop screw per the directions above and then set 
the other butterfly stop screw to flow the same amount. 

Adjustment 3:    Adjustment 3 is officially to position that lever against its own stop, take up all the backlash in the 
coupling under the butterfly stop screw, then tighten that clamp back down.  However, it moves in snaps -- you’ll never 
get it to clamp down exactly where there is no backlash.  So, hold the butterfly against its stop screw and move the 
lever towards its own stop until it snaps as many times as it’s going to.  Tighten the clamp down.  When done, the 
linkage at this corner of the engine should have a bit of backlash:  move it one way and it opens the butterfly, move it 
the other way and it sits on its own stop, and there should be a little movement in between.  Again, make sure the stop 
in this linkage isn’t what’s stopping the butterfly itself. 

Adjustments 1-3 should be performed on both sides of the engine, and obviously everything should end up as close to 
the same from one side to the other as possible.  If nothing seems to feel the same way on the second side as it felt on 
the first, you probably should start over and do them both again. 

Adjustment 4:  Adjust the throttle crossrods.  Obviously, you will need to put them back on for this procedure.  
Graphite lubricant is recommended in the ball joints, but it is not recommended that you lube them now; the powder 
might interfere with the adjustment.  Rather, adjust the crossrods then snap them back off to apply a little graphite. 

Position a spacer 2mm (0.079”) thick between the turntable and its idle stop.  A 2mm allen wrench works if you have 
one, but whatever you use try to make sure it doesn’t fall down into the nether regions somewhere.  The spring tension 
will hold a 2mm allen wrench in place during this job, but just the same it’d be a good idea to either tie a string to it or 
at least have the other end over the turntable pedestal itself rather than hanging out over the cliff. 

Loosen the locknuts on the crossrods and adjust each link such that the butterfly is just starting to open with that 2mm 
spacer in place.  The thread on one end of each crossrod is LHT, so with both locknuts loose you can just turn the rod 
itself to make it longer or shorter.  They even provide a knurl to help you turn it easily. 

“Just starting to open” is a bit tricky; if you have good light, you can carefully watch the lever lift off the stop screw on 
the butterfly.  However, a better idea is to install that .002” feeler gauge between the lever and the stop screw and allow 
the return spring to hold it.  Position it so it’s hanging over the throttle body rather than straight up.  Now turn the 
crossrod just until the feeler gauge falls down against the throttle body.