Idle stutter on the XJ - what's goin on??
HI,
Sounds like one cylinder not quite firing cleanly.
Try a dynamic compression check, removing one injector cable, so the injector doesn't fire, at a time to see if all cylinders have the same fall off effect on idle speed. You may get ECU fault codes saying injector fault when you do this. Similarly you may get mixture or other faults. So don't do it for long.
If it is one cyl running low it will have less effect when you cut off its fuel. Sometimes you can hear the uneven double "beat" in the exhaust when you disco a good cyl, the beat goes single and even when you have the suspect cyl.
Once you know which cyl, you can concentrate on finding the problem with it.
Try swapping plugs/leads ( at both ends) to see f fault follows the plug.
Try swapping injectors around to see if the fault follows the injector.
I had a similar prob with a falcon 6, was sticky injector that didn't fix after cleaning. A new injector solved the problem. Of course check all other things first, sounds like you have, but most likely idle-only problems are small manifold leaks.
To check for manifold leak, get an oil can with long squirter. Squirt engine type 20-50W oil along the intake manifold gasket all around the suspect cylinder. The oil will clog the hole and slow the leak to fix the problem temporarily. Dealer already replacing it means just as likely to leak as not in my experience.
Worst case could be a slightly burnt or sticky valve ( or hydraulic lifter ). To check this do a compression check the traditional way using guage in spark plug hole etc. One cyl down indicates mechanical problem.
Idle problem is generally not fuel pump related as fuel delivery is slow, and pressure required is low. Pump problems usually occur at high revs/load.
Disclaimer: Of course this is non-professional advice only. Check with a professional if you are unsure or not confident this advice applies to you.
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