Jaguar XJ-S. Manual - part 79

 
 

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There’s not much good reason to do this.  Perhaps you have an idea on how to make it easier to get to the spark plugs.  
Or, maybe just for a symmetrical look, you’d like the front 3 on each bank to face rearward and the rear 3 on each bank 
to face forward. 

 

 

AIR INJECTION 

 

The tubes on the air rails insert into holes in the intake manifolds leading some to believe it’s somehow involved with 
the intake system, but it’s not; the lines go through the intake manifold mounting flanges into the heads and terminate 
within the exhaust ports.  The purpose of the air injection system is to provide oxygen in the exhaust stream to enable 
the catalytic convertors to work. 

 

AIR INJECTION PUMP:  Roger Myers reports that the air pump on the XJ-S can be replaced with a GM unit. Take the 
old one to your local parts shop and ask them to give you one just like it.  Chuck Sparks says that A-1 (a rebuilder) lists 
it as part number 32128. 

 

GETTING THE AIR INJECTION SYSTEM TO ACTUALLY WORK:  John Napoli adds, “I have had the 
opportunity to see the air rails from Jag engines -- both 6 and 12 cylinder versions.  Very often, the cars have brand new 
air pumps and diverter valves and hoses -- so someone spent a good penny replacing all that stuff.  And on every one, 
the air rail tubes (where they stick into the head) were completely coked up.  You would need some really ambitious air 
to get through all this!!  Moral of the story:  if you are going to maintain your air pump system, don’t forget to do the 
obvious and simple and remove and clean out the air rail tubes.  Otherwise you are just wasting time and money.” 

You’ll also probably need to clear out the passages within the heads.  There’s nothing complicated about those 
passageways, they are straight shots into the exhaust ports.  You can clear them out with a drill. 

 

AIR INJECTION PUMP CONTROL:  Roger Bywater says, “the 2-3 seconds delay on the diverter valve was because 
of a controlled leak across the actuator diaphragm which allowed the valve to progressively reapply air injection while 
the manifold was still under high vacuum.  I seem to remember that removing the neoprene cover nipple from the 
actuator stub pipe prevents this leak from building up a balancing vacuum on the reverse side of the diaphragm, thereby 
totally disabling the air injection system.  We sometimes made use of this to take a before-catalyst gas sample from the 
air rails, fittings having been added for that purpose. 

“In fact there can be more to this diverter valve business that is not readily obvious and it can have advantages even 
with over-run cut off.  In the over-run condition the injected air from the pump becomes quite a high proportion of the 
total through-put so if no fuel were present, as would be the case with over-run cut off, the catalyst would lose 
temperature quickly.  Having a diverter valve to temporarily dump the excess air gives the catalyst an easier time, with 
less steep temperature cycling and helps to maintain catalyst effectiveness when the fuel reinstates.” 

 

AIR INJECTION PUMP WASTE OUTLET:  On the H.E., the air pump only delivers air to the exhaust system during 
warmup; once the engine is warm, it wastes the air into the right side air filter housing.  Michael Aiken points out that it 
is plumbed “to the engine side of the filter.  Unfiltered air is being pumped directly into the engine - and this is very 
dirty air coming out of this pump!  I would recommend that the wastegate tube be removed from the air cleaner housing 
and the opening in the housing blocked.”  Aiken adds that the filth coming through this line was actually visible on the 
inside surface of his air filter itself.  He also points out that, in addition to the dust that the air injection pump may be 
pumping into the intakes, the pump itself is junk and may start pumping bits of itself into the intakes at any time -- rust 
particles, bearing bits, whatever. 

 
 

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It’s possible that the waste line was routed to the filter housing to reduce noise, but if noise is a concern it should 
nevertheless be rerouted to the intake side of the filter housing, not the engine side. 

 

AIR INJECTION NON-RETURN VALVE:  Roger Bywater:  “The reason for the check valve in the air rail was not 
because exhaust back pressure could exceed the pump pressure, but to prevent exhaust backflow into the pump if the 
drive belt should fail.” 

Bruce Battles says that the Jaguar non-return valve “without the T-splitter is a GM part (check valve) Part # STI AV7.  
Just keep your T-splitter and install on the new GM check valve.”  Chuck Sparks provides a couple more part numbers: 
 Tomco 17001 and Std Parts AV-7. 

 

AIR INJECTION SYSTEM REMOVAL:  Removing this system is not recommended, as this would be a modification 
of an emission control system and would be illegal in many areas.  Too bad, too; this system accounts for a great deal of 
plumbing under the hood as well as a belt-driven load on the engine.  And on the Digital P cars, it only functions when 
the engine is cold -- the pumped air is wastegated after the engine warms up to prevent interference with the operation 
of the oxygen sensors. 

Eliminating the load would be easy:  just gut the air pump so it only serves as an idler pulley.  The existing check valve 
will prevent any backflow from the exhaust tracts into the gutted air pump.  And there’s little chance an emissions 
inspector would ever notice.  Eliminating the clutter and weight is more involved: removing the air manifolds and 
piping, plugging the holes in the intake manifold, and replacing the gutted air pump with an idler pulley.  The emissions 
inpectors are more likely to notice this if they look under the hood.  None of this will affect a tailpipe test, though, since 
such tests are typically done with the engine warmed up. 

If you chose to install an idler pulley in place of the air pump, there are lots of pulleys available for use on automobiles; 
you can ask for an idler pulley in any auto parts store and be presented a choice of several.  You’d need to fab some sort 
of bracket to position it, and remember that you’ll need to adjust the belt tension with it. 

The neatest pulley installation would result from using genuine Jaguar parts.  Peter Smith: “I removed my air injection 
system and used a jockey pulley to carry the V-belt.  These pulleys were standard equipment on English cars which did 
not have to meet emission requirements in the early days and some of these jockey pulleys may still be available in 
wrecking yards in England.”   

The 1987 Jaguar Parts Catalogue lists the part numbers: 

 EAC4185 

Pulley 

 C37886 Hub 
 EAC3042 

Bearing 

 C23128 Setscrew 
 EAC3041 

Bearing 

Holder 

 C37875 Bracket 

The pulley used as an idler is the very same part number that’s used on the air pump -- so you can skip buying 
EAC4185, just use the one you have. 

You can also probably skip buying the bracket C37875 if you’re willing to add a couple of inches of spacers around the 
pivot bolt for the air pump. 

John Napoli offers a suggestion for plugging the air injection holes in the intake manifold: “Buy 1/4” nail anchors from 
Home Depot.  They are aluminum and look like a large pop rivet.  Get them 1” long.  Cut them back to 5/8” long with a 
band saw or hack saw (only cut the body, don’t worry about the ‘nail’).  Put the two o-rings from each air tube on the 
shank of the anchor.  Smear on some engine-grade silicone.  Insert into the manifold and hammer down the nail all the 
way.  Omit the ‘plates’ and retighten the intake manifold to the engine.  Looks great and works great.”  This mod is 
likely to require an extra washer or two on each stud, since the studs may not be threaded far enough to tighten the nuts 
down on the manifold flange with the plates omitted. 

 
 

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Napoli:  “Here is the info on the nail anchors I used -- the ones I got at Home Depot: 

 

 

Star Anchors and Specialty Fasteners 

  1/4" 

by 

1" 

 

 

2615-32178  (part number) 

 

 

31916 26096 (number under bar code) 

“This is for a box of 40.  Enough for 3 V12s and 1 Opel Kadette.” 

Michael Aiken’s local Home Depot apparently carries similar anchors in a different package:  “"Red Head" light-duty 
anchors.  They are 1/4" x 1" hammer-set anchors used for anchoring to concrete.  They are easy to install just as John 
says.  They come in a package of 14 -- perfect for a V12 as long as you don't screw up more than two.  I have two 
spares.  The part number is 11234.  They are in the fasteners dept.” 

An addendum to Napoli’s idea: While Home Depot only carries 1” anchors, 3/4” long hammer drive anchors are 
available at some other hardware stores, usually in the bin right next to the 1” ones.  Since the flange on the intake 
manifold is about 7/8” thick, these anchors can be used without cutting them shorter. 

If you set about removing pump, valving, plumbing, and rails with the engine in the car, you will find it very difficult 
indeed to remove the pipe that goes from right behind the air pump along the bottom right side of the engine and 
emerges behind the turntable.  Some have resorted to ripping this pipe to pieces to get it out.  The easiest solution is to 
simply leave it there.  If you ever have the right side exhaust manifolds off for some reason, you can remove it then. 

 

REPLACING THE AIR PUMP WITH A GM ALTERNATOR:   If finding or fabricating an idler pulley to replace the 
air pump is too fiddly for you, note that installing a GM CS130 alternator in place of the air pump is easier and 
probably cheaper.  This replacement is described starting on page 577.  The GM alternator not only serves as an idler 
but also eliminates the need for the Lucas alternator and its V-belt in the process. 

 

AIR INJECTION -- 6.0 ENGINE:  Charles Randle says the air injection system in the 1993-on 6.0 engine is different; 
“Suggestions like do away with the system with an idler pulley is not an option.” 

 

 

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). 

 
 

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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 from above 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. 

Jaguar provides four threaded holes in each header.  In later cars, 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.  Rob Warnicke says his ’73 XJ12 had studs in all four holes per downpipe, eight per bank: 
“I don't know when the changed the set up.  I was surprised when I tore into my 87 and only had half the hardware to 
deal with.  The new flange and the old flange are essentially the same, with the new one having two fewer ears on 
opposite "corners".  They work the same, just with fewer bolts.  One would think that since Jaguar dropped it down to 
two per exhaust manifold, you could delete two on the older cars as well. ” 

 

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 (see page 341).  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.