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So my 1996 Jaguar XJR is just stopping at random. It happens when the car is doing low RPM, like when I am coming/stading at a stop light.

Any ideas on where to start to troubleshoot this problem?

UPDATE: Car works fine on higher revs - as long as I have the gas pedal pressed just a little bit - no problem! However, sometimes, even if I am driving like 50km/h, and let go of my gas pedal, the rpms just drop to zero, and the car gets really quite. Sometimes I find that dropping the car into neutral helps (yes, it's automatic gearbox), as the rpms then level off at about 2k.

NOTE: I would like to say that the idea of SO car site is an amazing idea!

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3 Answers

From your description, I would second cleaning the throttle body but more importantly get the idle control valve checked out. If that one is a bit "lazy" (usually because it's clogged), closing the throttle fully might starve the engine of air for long enough to die before the idle control supplies enough air.

I'd start with checking that the throttle position sensor has a working idle indicator (usually a switch) and that it's adjusted correctly. If the TPS doesn't signal "idle" to the engine when the throttle body is closed, that's a problem right there.

I'd also look at the idle control valve itself and check if it's possible to clean that with some throttle body cleaner. It might just be a little sticky.

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Does it idle properly at other times, or dies it die every time you let the revs drop? Does it work properly at higher revs, or is it lower on power than you might normally expect? Does it seem 'lumpier' than usual?

Normally a car not running well at idle is caused by a weak or mistimed spark, or poor fuel supply.

Have you checked the spark plugs recently? Make sure they are in good condition and are correctly gapped. I can't remember what sort of ignition system jags of that age used, but if it is adjustable, check the ignition timing. If it has a distributor, replace the rotor arm and distributor cap, and check or replace the HT leads. If it has individual coil packs, check they are all working properly - a failure of one or two could cause it to drop out at idle while still seeming to work properly under load. Clean the injectors and/or throttle bodies, and clean or replace the fuel filter.

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Thanks, updated my question! I also had a few suggestions that it might be fuel pump before - I guess I'll have to go to mech to check it out. –  drozzy Jan 9 '12 at 11:13
    
Yup, fuel pump is certainly a possiblity. Another thing I have just thought of is that the idle control valve might be malfunctioning, which would also give the symptoms you describe –  Nick C Jan 9 '12 at 13:36
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My alfa romeo used to do this - turned out it was a really dirty throttle body - never found out why it got so dirty but cleaning it fixed it instantly.

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What's throttle body? :-O –  drozzy Jan 9 '12 at 11:11
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@Drozzy, the throttle body is basically an air valve that is connected to the gas pedal. Push down on the pedal and the valve opens. Get off the pedal and the valve closes down to the idle setting (very slightly open). If your throttle body is very dirty, that idle setting may not be providing enough air for the car to keep running. –  Bob Cross Jan 9 '12 at 12:43
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