Jaguar XJ-S. Manual - part 67

 
 

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HIGH IDLE SPEED:  If your idle is too high and you’ve screwed the adjuster in all the way and it won’t come down, 
problems with the Auxiliary Air Valve are by far the most common culprit -- see below.  There are other possible 
causes, though, including a vacuum leak, butterflies sticking and not closing properly, butterfly stops not adjusted 
properly, or your overrun valves are opening when they shouldn’t.  There’s even one possibility involving a solenoid 
valve installed in the cruise control during a recall; see page 669. 

When the idle gets above 1800 rpm or so, the overrun cutoff feature kicks in and shuts the fuelling off until the rpm 
drops below 1100 or so, then it turns it back on.  The resulting cycling of the engine will get your attention!  People get 
very concerned when this happens, thinking it must be something far more serious than simply the idle speed is set too 
high, but that’s all it is. 

 

AUXILIARY AIR VALVE:  The auxiliary air valve (also called an “extra air valve” in Jaguar repair manuals) is in the 
same housing that the idle adjustment screw is on, and is supposed to open when the engine is cold to keep the idle up.  
It contains a bulb similar to those found in thermostats that protrudes down into the coolant rail on top of the B bank 
head.  A rod from this bulb pushes a piston up a cylinder against a spring, covering an opening in the side of the 
cylinder that allows air into the engine. 

Although they share the same housing, the idle adjustment screw and the cold idle function of the AAV are separate.  
The screw obstructs one passage allowing idle air into the manifold, and the piston controlled by the bulb obstructs a 
different passage allowing idle air into the manifold.  Hence, if the cold idle piston jams wide open, closing the idle 
adjustment screw will still usually not bring the warm idle down within reason. 

The bulb may look similar to a thermostat bulb, but it is not the same; see the description of the operation of a 
thermostat bulb on page 205.  A thermostat goes from fully closed to fully open in only a few degrees; the AAV needs 
to move gradually from open to shut over a range of 80°C or more.  So, where the thermostat bulb has wax in it that 
changes phase, the AAV bulb contains an oil; it probably works by simple thermal expansion.  For more on this, visit 

 

 

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

 

Michael Aiken says, “I believe there are many versions of the AAV depending on many variables and the correct one 
for your model is necessary.  My parts manual (which is for 87 to 89MY) shows no less than four part numbers for the 
extra air valve for the V12: Two for up to SN 8S.57761 and two after 8S.57761.  The other variable is emission code.”  
Mike Morrin calls that “an example of the later parts books not showing the details of early cars.  The June 1980 parts 
book shows C42164 used initially on all cars, then progressively replaced for various markets between 8S.5203 and 
8S.11800 with EAC1405 and EAC1590, which were in turn superceded by EAC2273.  Interestingly, EAC2273 did not 
supercede C42164.” 

Based upon inspection of several part number AAV’s, it is apparent that the relationship between temperature and 
piston location is the same for all.  The bottom end of the bulbs are all marked “-30º”.  The only difference between part 
numbers is the configuration of the opening in the side of the cylinder that the piston slides over.  By shaping this port 
properly, the engine designer can provide whatever airflow/temperature relationship he needs.  The port shapes used on 
Jaguar V12’s actually vary considerably; the web site mentioned above includes photos and dimensions of ports from 
various AAV’s. 

There are two common failure modes for the AAV; the piston gets stuck, or the bulb gives out.  If the piston gets stuck, 
possible solutions are obvious: disconnect the outlet hose and spray a bunch of solvent or carb cleaner into it, or take it 
off the car and soak it in solvent.  Mike Morrin says, “Going along the non-destructive path, if you look down the bore, 
you will see some smallish holes drilled in the piston.  I managed to fit a long self-tapping screw into one of these, and 
used it to pull the piston up against the spring a few hundred times, while applying WD40 to wash out the crud.  It 
seemed to work, and I have had no problems in the following 12 years.” 

If the bulb gives out, chances are it will do so gradually, and the result will be that the piston sits lower and lower at a 
given temperature.  Craig Sawyers, who lives in the UK, hot-water-tested a new AAV and found that it was totally 
closed by 80ºC.  He also tested an old but “working” second-hand AAV that appeared to be of identical construction, 
and it had the exact same sort of characteristics except shifted upward in temperature; if you extrapolate based on the 
positions he measured up to the boiling point of water, it wouldn’t fully close until nearly 110ºC. 

 
 

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Since a lower piston opens the port more, the cold idle will get higher and higher as the bulb gets weaker; when the port 
is no longer fully closed at operating temperature, the warm idle will start to rise.  For a while, this can be countered by 
screwing in the idle adjuster, but eventually the adjuster will be all the way in and the idle will still be too high. 

To make sure the AAV is the problem, remove the left side air filter cover and element, start the car and let it warm up, 
and check how much air is entering the hole where this valve is connected.  You might even try covering the hole 
entirely; since the warm idle air also goes through this opening, the engine RPM should fall to below a normal warm 
idle or even stall, although it will be sucking air through the distributor vent scheme like crazy.  If the idle drops a little 
but remains high, your idle speed problems are probably due to something other than the AAV. 

Based on observations of several AAV’s, it appears that the condition of the bulb can be easily verified: remove the 
outlet hose, and measure from the upper edge of the outlet fitting down to the inside bottom surface of the piston 
(which has several holes in it; make sure you’re measuring to a surface between the holes).  This dimension should be 
70mm at room temperature.  If it’s more like 72mm or more, it’s weak. 

Note that looking into the inlet fitting at the port might not be a good indicator of bulb condition.  Some of the port 
shapes are fully open at room temperature, while others don’t fully open until freezing.  Boiling won’t necessarily help 
either; most AAV’s are fully closed by 80ºC, but at least one type still has a portion of the port uncovered and 
apparently modulates the idle speed at operating temperatures. 

It is considered good practice to replace thermostats whenever the engine has been overheated.  Considering the 
similarities of construction of the AAV, it probably should be replaced whenever the engine is overheated as well.  Of 
course, a new AAV costs one  hell of a lot more than a new thermostat.  Just the same, overheating is only too likely to 
be shortly followed by AAV trouble; there are two seals in the base of the AAV around the bulb that are made of 
nitrile, and they get just as hard and brittle as a nitrile O-ring when things have been hot.  There is a sleeve inside the 
bulb that is probably nitrile as well. 

Would you install a used thermostat in your car?  Of course not.  The idea of installing a used AAV may be just as 
misguided, even though they are atrociously expensive.  Philip Jones says, “I spent a day pulling 4 off a local breaker's 
stock and found none satisfactory.” 

 

AUXILIARY AIR VALVE -- REBUILDING:  If you need to replace the thermal capsule in your AAV, the biggest 
challenge will be finding a suitable replacement bulb.  As mentioned above, a bulb from a thermostat will not provide 
the same response.  Richard Guevin suggests you check: 

Corea Electronics Corporation 
138-5, sosabon-dogn, sosa-gu, 
puchon-city kyonggi-do, Korea 

Zip Code: 422-233 

ph: 82-32-346-3665 

 

FAX: 

82-32-346-5326 

http:://www.coreaelec.co.kr

 

e-mail: cecgj@netsgo.com 

Guevin specifically suggests the item shown at 

http://www.coreaelec.co.kr/viewproduct_2_e.html

. 

Of course, you might opt to use a thermostat bulb anyway.  Rather than holding a nearly constant idle during warmup, 
the idle will gradually rise until the thermostat temperature is reached, and then it will quite suddenly drop to a warm 
idle.  If this is acceptable -- and it certainly is preferable to an OEM bulb gone bad, in which the idle never comes down 
-- just try to find the coldest thermostat bulb you can, so the idle drops before operating temperature rather than at 
operating temperature.  And make sure the thermostats in your engine are hotter than the bulb in the AAV. 

On the series of web pages starting at 

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

, procedures are presented for 

rebuilding the AAV.  If at all possible, review this site prior to tackling the job; the photos will be helpful.  The short 
version of the procedure:  Put the AAV in a press and push the outlet fitting down until you push it through the housing 
and push the baseplate out the bottom.  Press the bulb out of the baseplate upwards, which will destroy a peened-over 
edge and ruin a flat washer holding the bulb in place.  Applying sealant, install your new bulb and retain it using 
whatever you will trust to work; a 1/2” stainless steel flat washer with a series of tiny holes drilled around the outer 

 
 

265

edge held in place by a circle of #4 stainless steel screws threaded into the baseplate is a good plan, but make sure the 
holes in the baseplate don’t break through to the other side or they will cause a coolant leak.  After cleaning up the 
piston and other internal parts and making sure they move freely, reassemble by pressing the baseplate in the bottom 
and the outlet fitting/cylinder in the top.  Apply some Loctite 518 around the edge of the baseplate before pressing it in 
to help make sure it won’t leak coolant.  Make sure to stop pressing the outlet fitting at the right place, which should be 
where the inside bottom of the piston is 70mm below the top edge -- unless you’ve used some other sort of bulb, in 
which case you might want to stop pressing a little sooner, drive the car for a while and see how you like it, and if it’s 
not right just pull the hose off the top and tap the outlet fitting in a little farther until it behaves the way you want it to. 

Some suggest polishing a bit off the OD of the outlet fitting/cylinder to change the press fit to a slip fit, and then retain 
it with screws through the side of the housing.  This makes it a bunch easier to get apart next time.  A little sealant 
would make sure it doesn’t leak air, but really it wouldn’t be a big deal if it did; you’d just have to adjust the idle screw 
to compensate. 

John Ashcroft of Melbourne, Australia, says, “H&H Spares do a c/o for $90 AU, trade price; they have someone 
rebuild them so the bits are available somewhere.  Put one of their exchange units in about 5yrs ago and it's still 
working fine.”  See page 695 for H & H Prestige Spares. 

 

AUXILIARY AIR VALVE -- CHEAP FIXES:  If your AAV isn’t sticking but exhibits the “tired” behavior described 
above, Roger Bywater suggests a fix:  “It does seem that the range of movement can change with age possibly due to 
very slight leakage from the bulb.  In such cases it is quite feasible, after assessing the amount of correction required, to 
just press the top further into the casting body in a vice (with a socket or tube over the bulb) to get the thing operating 
back in range.  Obviously it has to be checked in boiling water before and after and it needs to be done with care 
because it is all but impossible to get it back if it goes in too far. 

“Don't go blaming Jaguar or Lucas for this valve and its foibles, although most of them run for years without trouble.  It 
was a Bosch invention.” 

Faced with a completely failed AAV, Jan Wikström simply started the car and let the AAV suck a small rag into the 
opening in the air filter housing to plug the port.  If the rag is small enough, it could conceivably plug the port without 
interfering with the warm idle adjuster.  Of course, if it’s too small it’ll pull it through the port and into the engine.  I’d 
suggest something around 2 inches square. 

With a little more disassembly, you can do a better job of plugging it.  Take the hose off the inlet fitting and jam a plug 
in there.  By making the plug short enough, you should be able to push it past the warm idle passage, which comes off 
the bottom of this passage, and plug the cold idle port only. 

Really cheap and easy: jam a cork into the hole into the opening in the air filter housing.  The problem with this is that it 
will suck through the distributor vent scheme like crazy to maintain idle.  Still, if you have a problem on the road, this 
might be a quick fix until you get home and can do something better. 

All of these last fixes, of course, completely eliminate any idle boost when cold.  Interestingly, several owners have 
reported that this doesn’t seem to hurt anything!  The V12 will idle at 300 RPM with no problem.  Some cars have 
another means of boosting the idle when cold, namely the supplemental air valve at the front of the right intake 
manifold.  This thing stays open for 45 seconds on US-spec cars and 15 minutes on UK/Euro spec cars to compensate 
for a disabled vacuum advance at idle; you might consider re-enabling that vacuum advance at idle (see pages 303 and 
134) while leaving the idle boost functional. 

Finally, of course, an entire separate cold idle scheme could be provided; John Napoli says, “just get one or two 
vacuum solenoids.  Wire to toggle switches, pipe the air thru an inline fuel line filter (or plumb to air box) and go to 
town.  These thingies are real cheap.  Get generic units unless you have some Jag take-offs in your junk box.”  For 
fancier work, you could install temp switches in the coolant circuit to operate these solenoids automatically.  By using 
different temp switches, the idle could drop in steps as the engine warms up and the solenoids shut off in sequence. 

 

 
 

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IDLING ON ONE BANK:  Kurt Hill has a very early XJ-S, and the AAV plumbing looks like the illustration in 
Section 19.20.16 of the ©1975 ROM or Fig. 3.52 in the Haynes manual -- which don’t resemble the plumbing in the 
later cars at all.  In this early layout, there is a tee on the outlet of the AAV, with one fitting connecting to the crossover 
pipe while another connects to a hose that goes forward underneath the left side intake manifold and connects into it 
from the bottom. 

Hill’s problem was that the car ran great at speed, but idled on the right bank only -- and flunked an emissions test big 
time because the EFI system was still squirting fuel into the left bank that wasn’t firing.  It turned out that a PO had 
replaced the hose from the AAV underneath the left manifold with generic hose, and since it has to make a 90º turn, it 
kinked as soon as manifold vacuum was applied to it.  Since this is not only the idle air supply to the left bank but also 
the crossover pipe, the left bank got no air.  Since this hose was hidden under the manifold, it was a difficult problem to 
locate; when the engine wasn’t running, the hose looked fine. 

The easy fix: replace the hose with a molded hose with a 90º bend in it, which is what the car originally came with.  
Better fix: install a metal spring inside the hose to prevent collapse.  Better fix yet: replace the hose with a section of 
copper piping with a 90º elbow, and connect it at both ends with really short pieces of hose. 

Those with later cars don’t need to worry about running on one bank at idle (unless they have Marelli ignition!), 
because the crossover pipe is no longer part of the AAV plumbing but a separate entity.  But any hoses under manifold 
vacuum are always cause for concern for sucking flat. 

 

OVERRUN VALVES:  If the throttle is closed at an elevated RPM, the manifold vacuum can exceed that normally 
found at idle.  Under such high vacuum conditions the fuel/air mixture being introduced into the cylinders, even at the 
proper ratio, can be so rarified that it cannot be reliably ignited with an ignition spark.  If that cylinder full of unburned 
mixture is then exhausted, it dumps quite a load of raw hydrocarbons into the atmosphere.  However, as Roger Bywater 
points out, an even worse result (from the car owner’s standpoint) is what happens when the mixture is later ignited 
within the exhaust system; it can blow the mufflers open.  This igniting within the exhaust is what causes the “backfire” 
upon deceleration that is characteristic of some cars -- mostly pre-EPA cars. 

“The purpose of the over-run valves is to limit the vacuum that the engine can generate on the over-run because it was 
generally considered that when vacuum exceeds about 22" Hg the charge density is so low and contaminated by 
exhaust residuals that combustion ceases to take place effectively.  The over-run valves therefore ensure that any 
mixture entering the engine is dense enough to fire.” 

At the front end of each intake manifold on the Jaguar V12 is either a blank-off plate or a little housing with a stub hose 
connection to the air filter housing.  The little housings contain the overrun valves.  They are a spring-loaded poppet 
valve that should open only when the manifold vacuum is higher than a set amount more than idle vacuum.  Basically, 
this limits the manifold vacuum to a level where ignition is reliable.  It also tends to make the engine feel as though it 
has no “compression” on deceleration -- basically, the throttle is being opened a little.  In fact, on some other 
automobiles, the same effect is achieved by physically opening the throttle. 

Does your car have overrun valves?  Apparently, it depends on whether or not your EFI system incorporates another 
method of addressing the same problem: overrun cutoff, in which the fuel injectors are shut off under deceleration (see 
page 273).  Bywater:  “When the EFI V12 engine was first introduced it had over-run valves and did not have over-run 
cut off.  Cut off was introduced in some markets in about 1977-8 and the over-run valves, being no longer necessary, 
were therefore deleted on the respective cars.  I cannot be sure from memory if US Federal emission cars were included 
but it is possible that they continued without cut off.  All manual transmission cars continued with over-run valves 
because they did not have cut off as it caused unpleasant driveline shunting.  Manual ECUs also gave slightly different 
low end fuelling being able to run at full throttle below the stall speed of a torque converter. 

“Cut off on the D-Jetronic system was effective at all temperatures unlike later Lucas Digital cars in which cut off was 
inhibited at low temperatures, necessitating the return of over-run valves.  

“I think it is correct to say that if a D Jetronic engine has over-run valves it would not have over-run cut off, unless of 
course someone has swapped the ECU at some time.