Jan 21, 2018

Idle hunting but only when warm


Brief history: replaced old fuel and flushed through with fresh stuff, replaced all spark plugs, checked all injectors are firing with a stethoscope (well they make the same tick noises). Car is from 1987 HE V12 with Lucas ignition


Looking for some help pleasse, symptom is that the engine would start and run fine when cold, idling around 1000-1200 rpm and looks to be balanced and smooth. Then as it gets warm after about 10-15mins, it would start hunting, oscillating between 500 - 1200rpm. Any ideas on where to check next?




I have the same problem on my 1989 V12 HE someone has said it could be the aux air valve but want to know more about it frist

Jan 27, 2018

This is a very common problem with all engines and can usually be put down to a very, very dirty throttle body, or more than likely, a duff auxilary air valve. It was a known fact, especially by Jaguar, that these were cheaply made and not up to the job.

Our own 4ltr convertible does it and I have a replacement unit to fit. I‘m not sure about V12’s, but the straight six is a fairly simple job. One electrical connector to pull off, two nuts/bolts and the old one comes off. Refitting is the reverse.

Where is it? On the rear side of the throttle body, which is very easy to see.

The valve is designed to open via a stepper motor when the engine is cold allow the ECU to adjust the correct air mixture...like a choke on older engines. As the engine warms up, the ECU tells the aux air valve to close via the stepper motor. Sadly, due to all the heat in the engine compartment, the motor sticks making the engine hunt. Eventually, the motor will close the valve and the car should run normally.

If it doesn’t, then the valve needs replacing.

There are some on sale on Ebay for later Six pots, but I’m not sure about other engines and what one will fit, you will have to do some research.

Hope this helps.

Martin (admin)

Feb 11, 2018

Hi Martin, thanks for the pointer. Checked the AAV the other day and it appears to be functioning correctly. I guess will double check it by blocking it when warm, and next is to check for any vacuum leaks


(just forgot to check back in with the forum)

on my 895.3v12he it was a little hole in a 90bend that comes of the aav replaced it (after it fell to bits in my hand ) with a new one and runs like a dream its always the little things that stop the big thing running properly

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