XJS V12 - erratic idle, loss of power - the answer is in the story, somewhere

The car: 1985 5.3V12 Coupe - 56K miles.

The key turns, a momentary buzz from the fuel pump and the start is without hesitation. All 12 are running as one and the idle RPM sits at around 1200RPM.

The first 4 to 5 miles is like silk, power and acceleration is faultless - and then, a tweak of the throttle and the first, slight hesitation. A stop at traffic lights shows RPM still at around 12-1300 RPM. The remainder of the journey is ‘blippy’, the hesitation becoming a little more frequent, noticeable and longer-lasting with every press of the accelerator. Destination is reached (phew!). Average fuel consumption from the trip computer went from around 15 to 11 by journey’s end.

The return is a more traumatic. After a clean start, the RPM is up at 1500RPM and the hestitation is turning into a progression of lurches within a few hundred yards of start. The best speed I can get is around 25 mph with RPM cycling between 500 and 1500 RPM. I reach the point where acceleration is almost impossible. I pull over.

After 2 mins I start again, RPM goes up to around 1500 then slowly comes back to around 1200, and off I go. Acceleration is a bit better, I can sit at around 45mph but there is a ‘blip’ with every press of the accelerator.

The final turn into my road requires slowing to around walking pace and as I lift my foot off the accelerator, the engine dies. Immediate restart - dead. A start 30 seconds later; a good start with the now characteristic 1500 RPM and I make it home.

So please, all you XJS Sherlock Holmes out there, what does the mystery suggest to you and what is the villain(s)?

Many thanks

I would guess your AAV is stuck open.

I don’t think a stuck AAV would cause the failure to run? It would just idle fast all the time. Which, since it seems to be doing that, may indicate an AAV problem or at least that the idle speed needs to be set.
My money would be on crap in the fuel sump and/or a plugged fuel filter.

I see that you are new to the list. Welcome.

What is the history of this car. Did you own it for years and it just started acting up? Or did you just get this car, have no history on, and just discovered these problems as you started driving it?

I think Greg and Bob nailed it. You have two very common but separate problems with these cars, a stuck Auxiliary Air Valve (AAV) that is causing your idle problems and fuel delivery issues most likely due to debris in your fuel tank clogging things up.

If you haven’t already done so please download Kirby Palm’s excellent “Experience in a Book” from Jag-Lovers (it is free) and read up on your problems. You can also seach the archives for “Auxiliary Air Valve”, “AAV”, and something like “fuel tank debris” and read the many posts about these problems and what others have done to fix them.

You may have to remove and replace your AAV with a rebuilt one but freeing up yours is possible. You will probably have to drain your fuel tank and remove and replace your fuel filter (it is in the trunk foward of your spare tire).


Might want to check your coolant temp sensor, too.

Many thanks for all your prompt replies. It looks as if the AAV will be the first port of call followed by the fuel system.

I’ve owned the car for 20 years but must admit to its only getting occasional use and the bulk of the faults seem to have appeared since the winter lay-up of last year.

Up until then I can now recall that the post start saw the RPM drop from 1200 at start back to the idle of 700 within about 5 mins - as advertised.

I did replace the fuel pump, filter and did take a look, although rather cursory, at the sump filter after the pump started whining after 20 mins or so into a trip. However, the new pump whined from almost day one and there was never any hesitancy until I did the work on that so I’m wondering if I disturbed some gunk in the process?

I did check the CTS and it appeared to be come up with the correct data.

I’m in the UK; does anyone know of an AAV refurb service?

Meanwhile, I’ll wait for the temperature in the garage to go up a few degrees before I get under the bonnet (hood).

Sure, you take it out, soak it in a gunk solvent (everyone here will have their favourite) and then put it back. Someone did post a link to a stainless steel modified version, but I would do simple stuff like this yourself, save your money for when things really go bad.


Common issue, which happened to mine, is that the thermal bulb gets ‘sluggish’ and doesn’t close all the way. So you’re getting way too much air at idle when warm.

In the book, I went the quick way of fixing it by whacking it properly so that it closes all the way once warm. But now it runs a bit rough during the first five minutes when cold, because it doesn’t open all the way.

Eventually, I will probably send mine to John John for rebuilding.

I can offer AAV rebuild service. You can try soaking your AAV or squeezing thermal bulb but it will not fix main issues with it. Usually AAV piston gets stuck due to galling and/or thermal bulb wears out and gets “slugish” with age and doesn’t close AAV all the way. Soaking or squeezing doesn’t solve any of the problems above and can be only a short term quick fix. I rebuild AAV’s with all new stainless steel internal parts and new correct spec thermal bulbs. Basically it will be better than new :wink:

Most of AAV’s I receive for rebuild people already tried “fixing” them in one way or another… I have seen squeezed bulbs, aav’s soaked in various fluids that eventually cause corrosion of internal parts, piston cylinder part hammered into the housing, I have received AAV’s with replaced bulbs from thermostats, air holes partialy or completely blocked with various bolts rubber plugs etc. and none of the above worked out well…

Thanks John for rebuilding my AAV!
My idle is now fine at cold start and when the car is warm.


Please would you send me your email address?



Update 1: After a helluvva job getting the hoses off, I extracted the AAV and gave it the hot water test. The cylinder is rising to close off the aperture as temperature increase; on cooling, it comes back down but it sits approx 1 mm above the lower edge of the aperture. I’m thinking that this would not make much difference in the great scheme of things.

A few questions, then: would it still be worthwhile having the AAV refurbed and is there anything else (obvious) at this end to look at before turning attention to the fuel pump/filter/sump tank?

Just an afterthought: the engine temp guage has never registered more than half way between C and N - any possible connection?

That 1mm let’s s lot of air in. But if it’s closing all the way when warm, you’re good. Mine was still open about 1mm when warm, which shot my idle up to 1200.

Properly working AAV should fully close at ~ 65°-70°c. It also depends on air hole arrangement as there are few, but at room temperature it should be more or less halfway closed.
Looks like that your AAV is “tired” but still operable to some extent. From your description I don’t think that AAV is causing your poor running issues… It might cause higher Idle though.

Sounds good.
The piston is supposed to be fully retraced around 0 degrees C.
Most important, as said before, is to fully close above 70 degrees.

I think though that your engine hesitation is a separate problem.
Sounds to me like either a fuelling problem, O2 sensors broken and ECU out of adjustment, broken CTS, either a stuck centrifugal - vacuum advance?

Actually it depends on AAV air hole configuration… some of them will be fully open only at ~ -25°c. So far I have seen 4 different air hole arrangements.
But as you say it is more important that it closes ~70°c

Well, after giving it a few more checks with boiling water, perhaps John_John is correct in his observation that it’s ‘sluggish’. After 4 tests, closer observation shows that it isn’t completely closingleaving approx 1mm visible at top.

Should the piston completely cover the aperture?

Yes. Here’s a picture of rebuilt AAV (left) next to old “slugish” but not stuck AAV (right) at ~70°c Note old unit is still open ~2mm

It depends on the aperture:


Thanks to both Kirbert and John_John. With regard to type, mine is the first as per Kirbert’s diagrams with the trapezoid aperture; and mine is definately the one on the right in John_John’s photo.

As I’ve got it out now, new gasket and hose ordered, and it is 34 years old, a refurb would not go amiss so John_John, would you contact me through the messaging system.

Next to-do then will be the fuel system components and the sump tank about which I have a question: in the engine-off situation is there fuel in the sump tank and if there is, what’s the best way of draining it? I’ve got clamps to facilitate removal of the filter and pump.