Engine won't restart when warm

[Holly1638]

Recently my 1991 5.3 V12 has been behaving badly. After a short run involving a stop or two during which it has started and re-started and ticked over normally, I will go back to it after parking 15-20 minutes and find that it will fail to start up, firing badly for some seconds before stalling, with the revs bouncing up and down eratically. Putting the car into gear has no effect. There is nothing for it but to leave the car and come back some hours later. It then starts and runs normally.

This is making trips to B & Q or anywhere else very fraught, so I would appreciate any suggestions as to where the problem might be. There is an automatic cut-out which sometimes starts operating when the car is idling (why, I don't know), but this causes a rhythmic bouncing of the revs from 0 to about 1000, and always stops when the car is put into gear. This new problem is I suspect somehow related to whatever mechanism governs the cars idling speed.

Richard Bancroft

[HerefordMark]

Richard,
It could be any number of things, but I would start with the basics - is the engine getting fuel, air and a spark when you try and start it? If it runs OK at other times, air might not be an issue, but I would check for blocked filters. Or fuel might be the issue. I am sure I read something about fuel pumps getting blocked when the car was hot (can't see why, but worth checking?). If not, then I would check for a spark - clearly the heat is causing something to break down - rotor arm, HT leads? Coils I think can also be affected by adverse heat.

If all this fails, it might be ECU/PI related. More difficult to check, but I would start with the variuos temp feeds to the ECU, including the coolant and air sensors - maybe they are giving a false reading when hot, preventing the ECU delivering the right mixture? 1992 cars also had a hot start system fitted - I know yours is 1991, but if it's a late model, this might be present. There's a temp sensor under the RH fuel rail which might be causing the problem.

I would also look at Dave Mark's website - there are a number of fault FAQs which might help, and you can always ask him for advice - he knows what he's on about.

Hope this helps.

HM

[Dimdip]

I've had something similar on my V12. It will sometimes be hard to start - turning over fine but just not catching for a couple of attempts. I've always put it down to a vapour lock in the fuel rails due to the underbonnet temp. I'm now leaving the bonnet unlatched with engine at idle for a couple of mins after parking and had no problems since.......but it has been cold weather:!:

[Holly1638]

Thanks for the suggestions. The engine does fire, but runs very eratically on start-up before soon stalling in the circumstances I've described.

On reflection, what I really need is someone to recommend a mechanic in London or the SE who knows the XJS backwards and will take a look at it. My regular mechanic happily services my old SAAB but regards the XJS with a loathing no doubt based on many years experience.

[Jon]

Not sure if this is any use but my 3.6 had a similar problem whereby it wouldnt re-start if left sitting for a while.. turns out it was the crankshaft position sensor on its way out, breaking down under the heat.. although in my case the engine either ran perfectly or not at all.

[apax]

Hi Richard - Try Eddie Spear, Jaguar specialist. He's knowledgeable, reasonably priced and honest - 01959 575120 (Biggin Hill, just south of Bromley).
Cheers, Archie.