Jaguar XJ-S. Service manual - part 14

 
 

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LEAK CONTROL VIA PCV:  In the July 1999 issue of Jaguar Driver magazine, Crispin Hales relates a story about a 
V12 E-type that had been converted from the original Zenith-Stromberg carburetors to SU’s by a previous owner.  The 
car leaked oil badly, and one by one all the leaks were corrected except for the rear main seal.  After procrastinating for 
years about fixing the rear main seal, it finally occurred to Hales that perhaps the PCV system was at fault.  Sure 
enough, whomever had converted the car to SU’s had removed the PCV system in entirety.  Simply installing a new 
PCV system caused the rear main seal to stop leaking. 

It’s unknown what the PCV system on the SIII E-type looks like, but it wouldn’t appear that the system on the XJ-S 
would help reduce leaks this well.  Of course, you should fix the leaks!  But perhaps it’d be nice to achieve the leak 
reduction benefits of a typical PCV system on this V12. 

It would be easy enough to make this system work like a typical PCV system: remove the PCV valve from the chamber 
on the LH air filter housing and plug that hole, leaving the crankcase vent connected to the air filter housing via the 
chamber.  Install the PCV valve into the crankcase itself.  That way, the PCV valve will draw air from the LH air filter 
housing into the crankcase through the metal mesh, through the crankcase, and through the PCV valve into the intake 
manifold. 

Of course, there’s no hole to connect the PCV valve to!  You’ll have to make one.  As far away from the existing vent 
as possible is good, but not essential.  You want to avoid any place where there’s a lot of oil splatter that the PCV might 
suck up, but most engines put it right in the valve cover so installing it in the RH cam cover should work just as well.  If 
you’re good, you can drill a hole in the cam cover that’s the same size as the hole in the chamber where the PCV valve 
was originally installed so you can install the same valve in the same grommet. 

Other options would be to find a way to connect it to the plug for the timing chain tensioner or to a half moon seal.  
Either way would avoid cutting on expensive metal parts. 

Remember that you’re not bound by the original PCV valve.  A check through the selection of PCV valves and 
grommets in an auto parts store may generate some ideas.  Some PCV valves have a 90° fitting on them, which might 
be helpful. 

If you manage to install the PCV valve somewhere in the right rear area of the engine, note that you should be able to 
take the tube connecting both intake manifolds to the PCV valve off and turn it around backwards and reinstall it, 
providing a ready connection to the right rear.  Of course, you can just connect up some hoses to the same fittings and 
route them anywhere.  Connecting to both manifolds is probably good, since you want to have the same effect on 
mixture on both banks. 

Obviously, it is of considerable importance that the owner take care to maintain the integrity of the crankcase 
containment; an opening into the crankcase will not only cause an oil leak at that location, but it will also allow air to 
enter and reduce the effective vacuum in the crankcase and thereby cause oil leaks elsewhere.  A classic location for 
such a leak on the Jaguar V12 is the timing chain tensioner cover (see page 46), but any opening into the crankcase will 
do it.  Make sure your dipstick is seated properly, make sure your oil fill cap has a good gasket under it, etc., etc. 

 

PCV FOR WORN ENGINES:  In the course of maintaining a slight vacuum in the crankcase, the PCV system must 
deal with whatever leaks exist to allow air into the crankcase as well as blowby from the piston rings.  With a very 
worn engine, the piston ring blowby may overpower the PCV system even if all the crankcase openings are properly 
plugged.  The EPA doesn’t really care as long as the air intake to the crankcase comes from the air filter housing; when 
the vapors overpower the PCV, the excess comes out into the air filter housing and gets pulled into the engine anyway.  
It just makes the inside of the air filter housing grungy.  The systems the EPA don’t like are the ones with a vented oil 
filler, where the excess vapors get blown all over the engine compartment and out the bottom of the car. 

Of course, you might not care for the oil leaks that result when the PCV system is overpowered.  It has been suggested 
that, even though an engine rebuild at this time is prudent, the addition of a second PCV system may help keep the oil 
leakage to a minimum until you can schedule enough time for an overhaul.  Back in the days of carburetors, adding a 
second PCV system would have been troublesome because you would have to figure out how to get enough fuel into 
the engine at idle when all that intake air was bypassing the carburetor venturis.  However, with EFI you really don’t 
have anything to worry about; just install it and the EFI system will meter the fuel accordingly.  You may have to adjust 

 
 

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the idle speed screw a bit.  Note that the piston/ring wear rates on the Jaguar V12 are so low that if you are considering 
this band-aid fix on an engine with less than 200,000 miles, you are advised to look for damage or leaks elsewhere first. 

 

PCV VALVE:  The genuine Jaguar PCV valve is part number C 44231, and the valve itself is stamped E7 AC 3848.  
This thing costs about $40.  That’s extortionate. 

Kelly Spongberg says, “I replaced ours with a generic jobber one made for older Ford V8 engines.  It is exactly the 
same size, and seems to work fine.” 

A quick review of the selection of PCV valves at a local auto parts store revealed that it may be difficult to find a PCV 
valve that won’t fit the Jaguar V12.  There are a couple that appear nearly identical, such as the Fram FV202.  There are 
a bunch more that are the same shape except they have a hose barb on the outlet end -- but since that end gets jammed 
into the grommet in the chamber on the LH air filter housing, you won’t see it and it’ll work fine.  There are yet a few 
dozen other PCV valves that appear the same as these except that they have an “F connector” on them that allows the 
connection of two hoses; you can just pull the F connector off and install it in the Jag.  There are even “generic” PCV 
valves that come with a selection of F connectors; throw all the connectors away and use the PCV valve as is. 

Of course, there may be a good reason why there are dozens of PCV valves available that all appear interchangeable: 
they may have different innards, like different springs or port sizes so they flow differently at different operating 
conditions.  Dave Osborne bought a valve that was visually nearly identical to the Jag valve for $3.  However, when 
installed the idle was 100 RPM lower than before.  Apparently the Jag valve has a stronger spring in it that holds the 
valve a bit open at idle, while the Ford valve has a weaker spring and is totally closed off at idle. 

Big deal?  Perhaps not.  You can simply readjust the idle.  And, considering the way the PCV system works on this car, 
it’s questionable if the PCV valve does anything of value anyway. 

 

 

Fault Diagnosis

 

 

MISFIRE:  With 12 cylinders, some people might not even detect a misfire.  An easy way to check -- as well as to tell 
which bank is acting up -- is to fold a dollar bill in half and hold it over an exhaust pipe outlet and listen to the flapping 
that results; a misfire is usually obvious.  Steven Draper adds, “According to the most recent Jaguar Repair Information 
Periodical, the official procedure is to use a one hundred dollar bill and send it to the dealer for evaluation.  
Unfortunately, the bill cannot be returned.” 

 

COMPRESSION CHECK:  First, a brief description of how to properly perform a compression check on any car:  The 
battery and starter must be in good condition.  All of the spark plugs should be removed.  Power to the ignition system 
should be disconnected, since an electronic ignition system may be damaged trying to fire with the spark plugs 
disconnected.  And the throttle should be held at least part way open, usually by jamming something in the linkage.  On 
a car with EFI, it would also be helpful to disconnect the power to the EFI system or fuel pump to prevent fuel flow.  
With a compression gauge fitted to one spark plug hole, the engine should be turned on the starter through several 
compression strokes, until the reading stabilizes at a peak value. 

On the XJ-S, it would be most helpful to have the type of compression gauge that screws into the spark plug hole and 
has a lengthy hose.  Trying to hold the press-in-place type on this engine is not easy.  Also, since the A/C compressor 
usually has to be removed to get at the front plugs, you will probably have to run the test with a dangling drive belt; try 
to position it so there is no tension on it, and the crank pulley can turn within it without driving it. 

On the US-spec pre-H.E. engine, the readings typically will be about 130-150 psi.  The US-spec 5.3 liter H.E. engine 
has 11.5:1 compression, and will read about 200-220 psi.  Note that readings will be lower at high elevations. 

 
 

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Keep in mind, however, that the absolute values are not as important as the relationship between them.  There are 
dozens of factors that could affect the absolute values (including the calibration of your gauge), so if your readings are 
all a little higher or lower than the above, don’t worry about it.  But they should all be nearly the same; one significantly 
lower than the others is not a good sign. 

If one cylinder reads low, it is customary to add a couple tablespoons of motor oil into that spark plug hole and test it 
again.  In theory, the oil will temporarily seal bad piston rings but won’t seal a burned valve, so this test may indicate 
the level of disassembly needed.  Of course, the oil probably won’t seal a burned piston or a hole in a cylinder liner, so 
the results are best taken with a grain of salt.  Either way, the head has to come off. 

 

LEAKDOWN TEST:  The leakdown test is gaining in popularity among mechanics, who feel that it gives a better 
indication of the actual quality of the sealing of the compression chamber than the compression check.  To perform a 
leakdown test, compressed air at a known pressure (usually 100 psig) is fed through a device with an orifice in it and 
into a cylinder via the spark plug hole.  The pressure downstream of the orifice is measured, and the leakdown rate is 
calculated as the percent which the pressure dropped across the orifice.  If the leakage out of the cylinder is very low, 
the pressure after the orifice will be very close to the pressure before the orifice, and the percent of pressure lost will be 
low.  If the compression chamber has big gaping openings in it, the main restriction in the flow will be the orifice itself, 
and the pressure after the orifice will be closer to ambient -- and therefore the percentage lost will be much higher. 

When performing this test, it will be necessary to lock the crank still.  The air pressure in the cylinder will try to turn the 
crank to BDC, but at BDC one of the valves is likely to be cracked open, making your leakdown readings meaningless. 
 Try to lock the crank somewhere between halfway up on the compression stroke and halfway down on the power 
stroke. 

Note that, when doing such a test on the Jaguar V12, it’d be a good idea to take the oil filler cap off the left cam cover.  
We wouldn’t want a well-sealed crankcase (hah!) to cause unwarranted favorable readings.  Also, just in case there’s a 
leak in a head gasket, the radiator cap should be removed to prevent pressure buildup in the cooling system. 

Kyle Chatman points out that if the leakdown test finds excessive leakage in a cylinder, it might be possible to 
determine whether the leakage is from the rings, the exhaust valve, or the intake valve by listening closely at the oil 
filler cap, the air intakes, or the exhaust pipe outlets.  And a bad head gasket might cause bubbles in the coolant 
(especially if it’s the center cap that was removed) or other motion in the coolant level. 

It’s entirely possible that the leakdown test is highly regarded simply because it gives results in percent.  It should be 
pointed out, however, that the percent has no real basis and is entirely dependent upon the size of the orifice in the test 
device -- and Randy Wilson says “and there is no such thing as a standard orifice size.”  In order to be able to compare 
the results of one leakdown test against another, it would be necessary to confirm that the orifices used were the same 
size and had exactly the same flow characteristics and that the same air pressure was used for the tests. 

Wilson adds:  “Next question is:  How much air leakage is acceptable?  This all depends on how big the cylinder is.  
The bigger the bore, the more ring area there is to leak by.  Typically, the valves are bigger, too.  Here we could really 
use a percent of leakage vs. base volume, but our tester is effectively measuring finite volume of flow.” 

There are other concerns involving leakdown tests.  For one thing, since the engine does not turn during the test, it 
really only gives an indication of the leakage at one piston position.  While this is usually adequate, it may fail to 
identify certain types of problems such as localized damage on a cylinder wall.  Perhaps the careful mechanic could 
slowly turn the engine over by hand while doing the leakdown test, and watch the gauge for variance in the readings. 

The leakdown test does have the advantage, however, that if a mechanic is using the same leakdown tester at the same 
pressure all the time, he can get a real good idea of just what condition cars are in.  While compression readings will 
always vary from car to car, the percentage readings from his trusty leakdown tester should give very consistent 
indications on cars in similar condition regardless of compression ratio or other variables. 

 

 
 

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HEAD GASKET CHECK:  Michael Neal suggests two methods of checking the integrity of the head gaskets.  First, 
remove the rubber hood that connects the PCV system to the engine just forward of the oil filler cap, and look inside it. 
 Milky deposits are an indication that water is getting into the oil, usually a sign of trouble. 

For a more definite check, Neal recommends removing the spark plugs, pressurizing the cooling system to 15 psi, and 
leaving it overnight.  Then have someone turn the starter while watching the spark plug holes.  Mist (or worse, a solid 
stream of water) coming out of a plug hole means it’s time for engine work. 

Of course, pressurizing the cooling system usually calls for a special tool.  It basically is an adapter radiator cap and a 
hand pressure pump; Stant is the common manufacturer and it runs $60 to $70.  In a pinch you could tee into the small 
hose on the center fill pipe and apply pressure.  

 

TAPPET NOISE:  Roger Bywater of AJ6 Engineering and formerly with Jaguar engine development, says, “In fact a 
recognised source of tappet noise on the V12 is excessive side clearance of the tappet in the aluminium carrier allowing 
it to ‘rattle about’.  Remember the XK used cast iron tappet sleeves and they expand and contract at the same rate as the 
tappet so the clearance remained more or less constant.  This does not happen on the V12 and the minimum diametral 
clearance is set by the need for a top size (high limit of tolerance band) tappet to not jam in a bottom size carrier bore at 
minus 40 degrees in a Canadian Winter.  The other extreme of a bottom size tappet in a top size bore could well be 
quite sloppy when fully warm and the way the cam moves it around can be very critical.  Cam profile, tappet clearance, 
side movement, rotation and rock-over at peak lift, as well as valve seating geometry, all come into what is actually 
quite a complex phenomenon.  For the record the range of diametral tappet clearances involved run from about 0.0005” 
to about 0.002” at room temperature.  I wonder how many engine builders have even thought about measuring such 
things?” 

 

OIL PRESSURE:  An oil-fed sleeve bearing, such as used in the main and connecting rod bearings of automobile 
engines, is an excellent device -- much more so than most people understand.  When the parts are rotating, the parts ride 
up on a film of oil, much like skimboarders skim easily across very shallow water and seem to coast forever.  When 
operating properly, the metal parts do not touch each other, and there is essentially zero wear.  The friction is entirely 
within the film of oil. 

This system doesn’t work at a standstill, however, the same way the skimboarder will sink to the bottom when he stops 
moving.  The entire reason engine bearings have a soft, replaceable surface is because they must ride on this surface for 
a very brief time at startup, before oil is pumped to the bearings and before the bearings establish a film to ride on.  The 
hard steel surface of the crankshaft should slide on the soft bearing with very little wear on either, but startups still 
account for the vast majority of normal bearing wear. 

The shearing action of the oil tends to heat it somewhat; there is very little heat generated from shear, however, and 
many cars get by without oil coolers.  The main cause of heating of the oil is by contact with hot parts, notably the 
bottom surface of the pistons. 

The pistons are likewise supposed to skim up and down the cylinders on a similar film of oil.  It doesn’t work nearly as 
well, though, since the piston stops at each end of its travel for an instant, and because there is a less positive flow of oil 
to this area. 

This system also does not work very well for the contact between the camshaft and the followers.  This is because the 
contact area is a very thin line rather than a broad area.  If one of the two parts were as soft as the crankshaft bearings 
are, the force at the contact point would quickly tear it up.  In the Jaguar V12, these parts are immersed in oil during 
operation.  This ensures they are adequately lubricated, even during startup since the oil stays there, but it also generates 
more heat churning the oil. 

There are two primary bearing failure modes in any engine that result from lubrication problems.  The first and most 
easily understood is excessive wear and damage due to lack of lubrication.  The second is bearing overheating due to 
insufficient cooling oil flow.  These two are very different; in the latter case, the bearing may have enough lubrication