V12 erratic idling issue

Hi there,

I have an erratic idling issue on the V12 HE.

On P position, the engine is running with idle starting from 500 rpm to 1500 rpm then back to 500 rpm and then to 1500 rpm …as a “sinusoidal behavior”.

I check the butterfly on each side, and I am at 0,05 mm.

May I have a pb with the AAV ? or some fresh air somewhere ?

I can’t figure out what is the cause of the problem ? May I check again the butterfly and throttle linkage and rods ?

thank you.


Classic symptoms of AAV stuck open. The extra air pushes idle high enough for ecu to cut fuel, then it falls and fuel comes back, and then yo yos between two idle rpms.

Check XJS forum, there is member named John John who rebuilds them very well.

Many thanks Greg for your help. :grinning:

Check for vac leaks.
I had one that was the dizzy vac advance diaphragm blown, and it was sucking air. Pinch the hose to check.

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I will do that Rob, I’ll let you know.

Yes, i assume you have no major vacuum leaks. That would also cause stuck AAV symptoms.

The gist is that idle is too high for an ECU perceiving a closed throttle, Greg - for whatever reason…

A third possibility is a failed overrun valve in the inlet manifold - it will let in extra air. A failed AAV should not give 1500+ rpms with a cold engine…?

Somewhat irrelevent; the xk has a microswitch to tell the ECU the throttle is closed - and with a faulty/maladjusted switch the engine will not rev above 1500 rpms…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

I don’t have more than 1500 rpm with cold engine, but exact 1500 rpm to 500 rpm and again and again…

And where is located the microswitch, and how can I check it ?

when I press the accelerator pedal, the engine exceeds 1500 rpm, but the engine is not working “smoothly” ? (I mean “working optimally”)

The cold engine has more ‘drag’/friction, Christophe - which is why the AAV is there; to add air to increase rpms/power. As Greg says; if the AAV is too open and stuck, or indeed too much air entering via other ways - you get the symptoms you describe.

The V12 hasn’t got the microswitch indicating ‘closed throttle’ like the xk - the V12 likely use the throttle position switch for this purpose. However, your problem is simply too much air in idle - the problem is to find the source. I assume that with the cold engine and rpms below 1500 rpms, the rpms were steady…?

You may try clamping the air hose to the AAV - this should drop idle to the level set by your correct throttle gap of 0,05 mm; 400 - 600 (?) rpms. Sort of confirming that the AAV, which incorporates the idle adjust screw, is the problem. Have you tried adjusting idle with the idle screw…

Salient point is that when rpms rise above 1500 rpms with the throttle closed, for whatever reason; the engine will cycle as you describe…

xj6 85 Sov Europe (UK/NZ)

Thanks all.

You have to know that I work a lot on the car. The car was parked for a long time.
After I check all the systems, changed a lot of old parts and renew all the fuel and electric systems, I crank the engine.

I discover that the potentiometer was worn. I changed the potentiometer with a new one.

So, it could be logic that the AAV is stuck after a decade of parking.

I would like to know if when I changed the potentiometer, I have to do something else precisely, there is an adjustment to be made ?

Thank you for your help.

Yes, you have to set it’s position so the ECU sees the right voltage at idle, a somewhat fiddly process but easy.
Details on Kirby’s book.

How did you discover the pot was worn, Christophe?

The workshop manual describes adjustments, but while the old throttle switch could be adjusted using an ohmmeter, the later, throttle pot required a potmeter adjuster gauge is required. But Kirby may have another solution - as Aristides implies?

However, as both versions have the same fuel overrun cutout; the original problem will remain. Cutting fuel above 1500 rpms with throttle closed is an intended, and worthwhile, feature - and should be retained. So you still have to pursue the cause of the high idle - and the AAV is of course the first suspect…

xj6 85 Sov Europe (UK/NZ)

Hi there,

I change the AAvV kit. No problem to do that.
I change the bushes on the both throttle shaft. No problem to do that.

Then I crank the engine, the idle was stable at 2000 rpm. I screw the adjustment screw on the AAV body. Nothing happened.
I screw again untill the end, nothing happened. Idle stable at 2000 rpm.

I slowly unseated the adjustment screw, and suddenly, the idle began to the old “sinusoidal” style, that I explain befaore.
From 500 rpm to 2000 rpm, then back again and so on.

I notice the following : when I engage the “D” of the gearbox or the “R” of the gearbox, with the foot on the brake, the idle is stable at 1000 rpm.

Please, do you have some advices ? Thank you.

@Frank I discover by testing electricity “working style”. And the part wasn’t in the correct measurment.

It certainly seems that you have excessive air entering the inlet manifold. When you replaced the throttle shaft seals, did you set the throttle blade gap? Have you made sure the throttles are closing completely?
A quick check of the AAV system is to simply plug the inlet on the left air cleaner base; idle should drop to ~500 rpm. Likewise you can check the overrun valves by plugging their inlets on each air cleaner base; idle should not change.
What is the condition of the crossover pipe at the rear of the engine? Is it possible that you have air leaks at the injector seals? You can check by squirting a little engine oil around each.
The crankcase ventilation system is another air leak path; the valves can gum up and stick.

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A quick note…

When setting the throttle gap to the required .002" make sure to check you feeler gauge blades. It is very easy for the .002" and .003" blades to stick together. You’ll think you’ve set the throttles to .002" but you’ve really set them to .005" ! You’ll then go on a crazy journey trying to figure out why your idle is so high.


First of all thank you to both of you.

For Robert : English is not my mother tongue, and I can’t see exactly what you mean by

  • plug the inlet on the left air cleaner base (?? ) what is the air left cleaner (air filter, air filter box ?) what is the inlet (a hose) ?
  • overrun valves ? (what is it ?)

Crossover pipe at the rerar is OK
Where is the crankcase ventilation system ?? the valves can gum up and stick. Where is this valve located ?

For Doug : I check throttle gap (I mean the space down the butterfly when throttle axes are disconnected) with 0,05 mm, not 0,02 ‘’. Is it correct.
I have a gap with rubs lightly on the adjustment blade. I think is OK.

Again thank you for your help. :grinning: I am so newbie in Jaguar world’s.

Sorry. Air cleaner= air filter. The base is the piece that is attached (bolted) to the throttle bodies. Toward the front of each base is a port that connects via a hose to the overrun valves on the front of each intake manifold. The left base has a similar port at the rear that connects via a hose to the AAV.
.05mm equals the .002” specification for throttle blade clearance.

I will check.

No problem for ventilation crankcase.

The crankcase ventilation valve is connected to a steel piping that attaches to each intake manifold; the valve itself is connected to the breather separator and the air filter box by rubber hoses.

So my last investigation and the car makes the same sinusoidal idle…now from 500 to 1500 rpm.
I have the impression that the adjusting screw on the AAV is inoperant… And it’s strange, because I need a metric tool 13 to move the screw. It’s normal ?

I’m sure for the gap with throttles, without rods connected (0,002 ‘’) and with rods connected (0,002 ‘’)
I’m sure with the rods (length, working) connected to the opener system of throttles.
I’m sure for ventilation crankcase (I mean the rubber is new, may I take out the rubber and see inside ?)
I’m sure for hose at the back of the engine (connected with bank A and B)

Now :
I don’t understand => after I have a gap of 0,002", I need to make a check with throttles closed, how ?
I don’t understand anything about the procedure to check air leaks with plug the inlet… and so on. Maybe pictures with details or drawings will help me. I understand all the english words, but not the sentence !! (I’m not very clever…)

I have to check leaks with injectors as well. But it’s not risky to put oil at the base ? You have electric plug there.

Thank’s a lot again for your help.