View Full Version : XJS V12 Problem

03-14-2002, 12:19 PM
I'm having trouble with the fuel injected V12 in my 88 XJS. Once it's been run until hot and then stopped, when I restart it the idle swings from 1,000 RPM to zero, back and forth, sometimes dying completely. It goes away after a short time and runs/idles fine.

I thought it was the distributor retard process that goes on during the first 45 seconds after starting but disabling the relay made no difference.

The engine had a major tuneup in the last 12 months and I can't find any loose wires or hoses.

It's got the AJ Engineering ECU and intake system that was installed during the major tuneup so I don't think the ECU is the problem.


03-14-2002, 01:48 PM
It sounds like fuel vapourisation to me. Is the engine running hotter than normal? Just a shot in the dark, but the heat from the engine (I owned an XK12 and the heat put out is quite something) can cause the fuel in the lines to vapourise, leading to "iffy" fuel delivery on startup. As soon as the system has built up enough pressure to feed the injectors properly, all is well. I always thought that the system was pressurised all the time, but I don't have the mechanical background to be able to verify that. Another thought is that the fuel pump is on its way out, taking time to build pressure. The vapourisation sounds most likely to me though. Can't AJ Engineering add any ideas? They must have seen it before.

Incidentally, if you need more input, join the BB at www.jag-lovers.org (https://www.jag-lovers.org) they have so many gearheads that you are guaranteed the right answer. Good Luck!


03-17-2002, 12:16 PM
I had a Detroit car with a similar problem and the fix was two fold. The throttle body was dirty and sensors were having problems. Generally with fuel injection, vapor lock or related fuel delivery problems have been eliminated. Paul.

03-26-2002, 11:08 AM
i had the same problem with my xj 4.2
the fuel rail needs to build up pressure before the fuel is delivered at a constant rate. check
all fuel connections for a air tight fit. and replace your fuel filter. i usually put the xj
in drive and turn the key if as i were starting it for about a 1/2 a minute. that turns on the fuel pump and pressurises the fuel rail. then i put it in park and really start it. pressure is already built up and i dont have idle jump problem.

04-06-2002, 05:46 PM
If anyone is still looking at this thread, I've solved the problem. It turned out is was a clogged charcol canister causing excessive vacuum in the fuel tank. At least so far so good......

04-07-2002, 12:41 AM
same symptoms in the xj 4.2 when the fuel return
valves (left or right) go bad. another symptom
is running rich.