Jaguar XJ-S. Manual - part 66

 
 

259

 

VAPOR RECOVERY SYSTEM -- CHECKING:  If your XJ-S has the Rochester valve, the first check is quick, 
simple, and clear: with the engine running, open the fuel cap.  There should be absolutely no “whoosh” or any other 
symptom of pressure equalization.  If there is a whoosh, either the Rochester valve isn’t working, or the system is 
plugged somewhere. 

The second test requires more judgement.  When the engine is shut off, the Rochester valve becomes a 2 psi valve, and 
a small pressure or vacuum can build up in the tank.  After parking for some time, a little whoosh is to be expected, but 
too much is too much.  Too long is not the concern; typically, a nearly empty tank will whoosh for a long time (lots of 
air space inside) while a nearly full tank results in a very brief whoosh.  The concern, though, is not the duration but 
rather the severity.  David M. Johnson says, “A huge whoosh and an oil canning sound means the valve is not working 
-- the vent is connected wrong, or the vent is blocked.” 

Another sign of trouble: no whoosh at all.  There is a remote chance that the fuel in the tank has remained at the same 
temperature since shutdown and hasn’t developed any pressure or suction at all -- but that’s really unlikely, especially if 
the car has been sitting in the hot sun.  Generally, if you don’t get a whoosh when the car’s been sitting a while, either 
the Rochester valve is stuck open -- which may sound acceptable but it’s really not, it’ll quickly saturate the carbon 
canister and then you’ll have fuel odors, not to mention what you’d be doing to the environment -- or there’s a leak in 
the system somewhere, which obviously could cause odors, fires, you name it. 

In the US, the duration of the whoosh involves the little trap door in the filler that prevents you from inserting a fuel 
nozzle dispensing leaded gasoline.  Since you can no longer buy leaded gasoline, nozzle notwithstanding, this thing is a 
relic of a bygone era and could be removed without concern.  But if it is there, it effectively restricts vent flow when the 
cap is removed, and may make a short whoosh into a longer whoosh.  Conversely, if the trap door is gone, a long 
whoosh may seem deceptively short. 

If your car doesn’t have the Rochester valve arrangement for some reason, whether or not the engine is running makes 
no difference; either way, there should be a small whoosh.  

 

BACKUP FOR ROCHESTER VALVE:  The entire point of the Rochester valve is that the tank should be freely 
connected to the carbon canister when the engine is running.  It is a simple matter to make sure that happens: just tee 
into the vent lines before and after the Rochester valve and connect up a solenoid valve to bypass it.  Connect the 
solenoid valve to a switched power source so it is energized whenever the ignition is on.  That will ensure proper 
venting when the car is running. 

To ensure proper venting when the engine is off, you can install a 2 psi valve in another bypass around the Rochester 
valve.  If you don’t have a 2 psi valve, you can probably get one from just about any car in a junkyard; most cars use a 
simple system similar to the pre-recall scheme in the XJ-S.  Or perhaps you can buy a Jaguar part, CBC6820. 

Of course, once you have the 2 psi valve and the solenoid valve working together, you could eliminate the Rochester 
valve.  If it jams open, that’d be a good idea. 

Obviously, another option would be to install a second Rochester valve around the original. 

 

FAILSAFE FOR VAPOR RECOVERY SYSTEM:  There are reportedly several different gas caps used on these cars; 
some OEM, some aftermarket.  Some of them are a total seal, but others have a pressure/vacuum relief valve built into 
them to serve as a safety in case the vapor recovery system gets plugged.  Good idea!  If your gas cap is a total seal 
type, you might want to replace it.  Note that if you have the pressure/vacuum relief valve type, it may defeat the 
whoosh test described above; you’ll only get a small whoosh when removing the cap even if the vapor recovery system 
is totally plugged. 

If somebody has drilled a hole in your cap, you definitely should replace it. 

 

 
 

260

FUEL FILLER UPGRADE:  The fuel filler on the XJ-S -- at least this author’s ’83 -- is the old style that turns 1/4 turn. 
 Not only is this a lousy design, but it’s also difficult to secure; Jaguar offers a locking variety, but believe it or not their 
locking gas cap is officially not supposed to be used in the US -- it doesn’t seal as it should and therefore won’t meet 
EPA emission requirements.  Roger Bywater says, “Most lockable caps have a tang which engages with the underside 
of the filler neck so the sealing arrangements are probably unable to withstand any significant vacuum or pressure and 
are just able to stop fuel sloshing out.” 

On later models, Jaguar made the door over the filler lockable, which eliminated the need for a locking gas cap. 

Walter Acker IV points out that it is a simple matter to ditch this dinosaur and install a modern filler from a junkyard.  
The filler on his ’76 is different than the later designs (see page 243), but the general idea should apply equally well to 
either type.  Just peruse the local junkyard until you find a car with a modern tighten-until-it-clicks fuel filler cap on a 
filler pipe that looks like it would work in the XJ-S.  Remove it, pay the junkyard the five bucks or whatever, then take 
it home and make it fit.  Acker used a Ford filler to upgrade his ’76.  “The Ford filler that I had put in is just soldered at 
a radiator shop.  I was not specific enough about the position of the Ford filler in the Jag neck to them.  You must 
explain where the bottom of the Jag unit is so that they can line up the Ford unit so that the drain holes in the Ford unit 
are in a proper position so that the fuel will drain into the tank if there is any leftover in the neck system.  The collar on 
the Ford neck is of the wrong size (smaller) and at the wrong angle for a Jag XJ-S so it must be removed completely so 
that the Ford unit will slip inside of the Jag unit. 

“I checked with the state of California smog reps and they said that it is not a legal modification so if it is detected by a 
smog rep then the car may not pass smog in CA.”  Damn shame these guys are so picky, since this cap upgrade will 
improve the car’s emissions.  “The smog police will give you a problem but the normal smog techs just remove the cap 
and see it is threaded and test it without ever calling the officials for clarifications.” 

“The one thing that I like is the fact that when you go to a parts store they have a new style of gas cap.  It is called a 
Prevent which is a pressure release type so that you can release pressure buildup before there is a problem when 
removing the cap for fuel filling.” 

Note that you need to pay attention to how high the new cap sits; you don’t want to go through the entire modification 
only to find that the filler door won’t close.  And having to hack a side off the new cap will reduce the esthetics of this 
upgrade considerably.  Note, however, that the tighten-until-it-clicks cap can merely be turned a couple more clicks if 
the problem is a corner or something that obstructs the door. 

Bernard Embden -- who owns a ’78, also with the earlier flanged-type filler pipe -- reports:  “A visit to the junkyard 
resulted in the removal of the fuel neck and cap from a 1987 Ford (hacksawed the fuel pipe right out of the trunk- lucky 
no sparks).  I cut approx 2 inches from the filler tube, and the same from the Jaguar fuel filler pipe.  Keep your existing 
flange that mounts the pipe to the body.  You will need to modify it somewhat.  Make your cuts dependent on whether 
you will keep the existing flange or not.  Off to the welder.  Two trips later (he had no way to pressure test the weld) 
and I was ready to install.  The Jaguar filler pipe is quite large, approx 2 inches I.D.  The Ford pipe is 2 inches O.D.  
This made the welding quite easy because one pipe fits inside the other. Remember to change the tank gasket (large "O" 
ring thingmajij).  No leak, no drips, no fumes.  One of those projects that made so much sense that I should have done 
this years ago.” 

Regarding finding the right one in the junkyard, Embden says, “Most filler pipes started small and stayed small.  
However some started larger and then became small.  On these you only need the first few inches so it doesn't matter.” 

This author made this upgrade, but my ’83 has the later style filler pipe with no flange.  I believe this makes the upgrade 
easier, since you don’t need to worry about the alignment of the flange when you have the two pipes brazed together.  
And, without a flange, if the cap ends up too high for the door to close you merely loosen the clamps and slide the filler 
pipe a little farther into the tank. 

David Low points out that the XJ40 has the tighten-until-it-clicks type of filler, and suggests that it may be adaptable to 
the XJ-S “if you’re handy.” 

A locking gas cap for the tighten-until-it-clicks type filler is readily available, and appears to seal just fine.  These later 
caps won’t ding the paint as badly when you drop them, either, since they are plastic and weigh less than the old metal 
1/4-turn caps. 

 
 

261

 

VAPOR RECOVERY SYSTEM -- MAINTENANCE:  The vapor recovery system, including the carbon canister, is 
normally maintenance-free.  Activated charcoal can absorb and release fuel vapor forever, it doesn’t wear out or get 
“full”.  However, there are filter elements within the canister (after all, it is an air intake) that may eventually get 
clogged, and the carbon itself may eventually get contaminated with fuel additives or other non-petroleum substances, 
so it is recommended the canister be replaced once in a great while. 

If you’re really a skinflint, the carbon canister can be opened and the filters replaced.  There are two, one at the top of 
the charcoal and one at the bottom -- you must dump all the charcoal on a newspaper or something.  Both filters can be 
neatly replaced with coffee filters.  The canister can be held shut with aluminum tape when reassembled.  

The carbon canister itself has an opening on the top to atmosphere, and three fittings on the bottom (note: the canister 
may be mounted upside down, so top may mean bottom and vice versa).  The fitting labeled T is connected to the fuel 
tank, via the appropriate valves and the vapor separator.  The fitting labeled P is the purge line to the engine.  The 
fitting labeled C is capped off; it originally was the connection for venting the float bowls of the carburetors, but they 
are history.  Note: if you fit carbs, do not simply open this fitting and try to use it; a screen has been omitted inside as 
well, and you will draw carbon granules into the float bowls. 

One of the possible causes of the common fuel odors is the location of the canister in the XJ-S.  On most cars, it is 
located in the engine compartment where any escaping fumes will simply be blown away by the flow of air through the 
radiator.  But in the location in the bodywork forward of the left front wheel, there is no such flow.  The vapors may 
gather in the bodywork and eventually work their way to the passenger compartment.  Of course, there aren’t supposed 
to be any fumes escaping, so this is a secondary problem.  However, it might be a good idea to connect a length of hose 
to the atmospheric vent on the canister and route it out the bottom of the car. 

Another possible failure mode is that the engine is not properly purging the canister.  If the engine is not drawing air 
through the canister when running, the canister will quickly become saturated with fuel and will cease to absorb, and 
any further fumes coming down the vent line will escape to atmosphere. 

 

VAPOR SEPARATOR:  In the XJ-S, there are several vent lines from the fuel tank.  They all are routed to a small 
vapor separator high in the bodywork to the right and above the fuel tank.  This small metal contraption is intended to 
allow most fuel vapor to condense and drain back into the fuel tank.  The vent line to the canister is routed from a point 
high in this separator, so fuel vapors must be very determined indeed to make it past this point. 

Note that the vapor separator has been blamed for many problems.  Since it is steel, it is prone to rusting.  If a hole rusts 
through it, fumes will be vented into the bodywork.  Also, rust particles may fall inside it and plug the tiny vent 
passages and hoses. 

 

 

Vapor Recovery System:  Hess & Eisenhardt Convertible 

 

VAPOR RECOVERY SYSTEM -- INCORRECT PLUMBING:  David M. Johnson owns two H&E’s, and reports:  “I 
found that on both of my cars the fuel vent systems were plumbed incorrectly (not done by my local dealer).  The 
carbon canister under the front left wheel arch is connected to a vent pipe from the fuel tank, and a vent pipe to the PCV 
valve next to the LHS air intake.  The two vent pipes were wrongly connected, i.e. the fuel tank vent was connected to 
the valves for PCV pipe (two valves operated by vacuum from the LHS manifold), and the PCV vent pipe was 
connected to the Rochester valve that should be connected to the fuel tank vent. 

“So what’s the big deal?  Well, the valves that would normally operate the PCV valve only open under vacuum when 
the engine is running.  When parked and sat in the sun the tank cannot vent!  I tested these valves with 10 psi and still 
the valves would not open.  The tank is now a sealed container with fuel vapor inside!  There are only two things that 
can happen: 

 
 

262

 

(a)  The tank will build up a severe pressure and will rupture in time due to fatigue at the weakest location.  A 
pressurized gas vapor canister in my car/garage does not give me a good feeling! 

 

(b)  If you are lucky, one of the vent pipe/fuel connections will leak to relieve the pressure and cause a fuel 
leak and/or a fuel vapor smell.  I am still not keen on this option with the car in my garage! 

“If the tank is connected correctly to the Rochester valve, the valve will open at approx. 2 psi to relieve the pressure in 
the tank, and also opens under vacuum.  This stops the tank venting under all conditions and hence does not overwhelm 
the carbon canister. 

“Of course my vent works perfectly now, no gas smells and no severe pressurization of the tank, just a small woosh 
when I open the tank cap.  I wanted to pass on this info to others, since I consider this a dangerous fault, in this case not 
a design fault but an incorrect installation by the Jag dealers on two cars!  Very ironic since the recall was to solve this 
problem.” 

“This recall was Hess specific (recall (C002) XJ-S purge kit (Feb 1990).  If you check the recalls on XJ-S 86-91 you 
will find there were similar problems with the regular XJ-S but a different fix.  The mod was estimated to be 4.35 hrs 
for later Hess cars to install Kit JLM2046 and conduct a fuel system integrity test. 

“The diagram (figure 6, page 8 of 23) showing the valves and carbon canister connections is not very clear because the 
fuel vent pipe connections are only partly shown, but the PCV pipe vent connections are clearly shown.  I deduced 
these pipes were incorrectly attached by testing all the valves to determine how the vapor system functioned.  I called 
the Jag dealer the next day to get a copy of the fuel vent recall, and hence confirm the connection was incorrect. 

“I do not think the two cars were fixed by the same dealer for the recall.  One car is from New York, and the other is 
from California.  That is why I suspect other cars could also be connected wrongly!” 

 

 

Throttle Linkage, Idle Speed Adjustment 

 

IDLE SPEED ADJUSTMENT:  On each butterfly housing, there is an adjustment screw that a lever contacts when the 
throttle is at idle.  These are not to be used for setting idle speed.  If they have been disturbed, the linkage must be 
readjusted as described on page 269. 

Underneath the rear end of the left intake manifold there is an aluminum housing with two hose connections; one that 
goes up to the back end of the manifold and one that goes forward to the air filter housing.  This contraption is called 
the auxiliary air valve, or AAV for short.  Just below the connection leading to the air filter housing is a bolt.  This bolt, 
believe it or not, is the idle speed adjustment.  The bolt itself obstructs an air passage, so the farther the bolt is 
unscrewed, the more the passage is opened, and the faster the engine idles.  It’s not real convenient to get to, but a 
ratchet and a long extension with a swivel at the end will do it. 

There’s a photo of the adjuster at: 

 

http://www.jag-lovers.org/xj-s/book/IdleScrew.html

 

On this author’s ’83 (as well as in the photo), the adjuster is a 12mm thread with a head barely larger than the threads, 
so the head requires a 13mm socket to fit.  However, Craig Sawyers reports that later versions (including replacement 
valves to fit earlier cars) use a metric threaded adjuster with a much larger head:  “The old one is indeed metric, M12, 
but metric fine thread.  It is however a special, with a 13mm AF head (standard is 19mm AF).  New valve 12mm over 
the threads, 1.75mm thread pitch and 19mm AF.  That is a standard metric M12 bolt.  Truly huge by comparison.  
Jag/Lucas have clearly economised by using a standard M12 coarse thread bolt with the standard head.  Adjustment 
sensitivity with the coarse thread is just fine.”