My idle seems to be increasingly lumpy and/or prone to hunting (feels like it is missing) but only when the car is fully warmed up.

Any thoughts?

 

I have the same issue with my '88. When warmed up and in drive at a complete stop, the idle drops to about 4-500 rpm and is all lumpy as you describe. Feels like it's on the verge of stalling. Hopefully someone has some advice..

 

Some folks on an XJ-S group on Facebook suggested it might be an air leak.

"for sure you have an air leak. So, the engine should be quiet, go looking for noise or use a propane (ask kirby) tank and spray gas around the motor. For example, the seals on your injectors could be leaking. Check the back crossover pipe to make sure it is fitted properly, it could be pushed into the hose such that the steel pipe into the motor is a proximity fit, only found by taking the unit apart. Anyhow, a hunting engine is an air leak. Oh, this is a good time to take up smoking.......turn motor off, remove a vac hose and have somebody blow a cigarettes worth of smoke into the hose and stand back, smoke will emerge from the leak. I had the power brake air pig leak in the passenger front wheel well, discovered this way, otherwise it wold take you a million years to figure that one out"

 

So, the hunting idle was a sticky but not faulty auxiliary air valve. Cleaning that seemed to clear up the hunting that was happening once the car gets out of cold start.

The rough idle was a failing primary ignition coil which was causing a periodic misfire on random cylinders. Replacing the ignition coil smoothed out the idle.

Thanks for all the feedback and suggestions.

Hopefully this will go into the shared knowledge about these cars.

 

My '95 XJS 4L idles roughly when warm. I've read lots of posts, and maybe the ignition coil(s) are a problem (car has 140,000 miles), but answer me this: (1) is there a "primary ignition coil" other than the six ones holding the spark plugs? (2) if the problem is within the six-coil system, how does one know that one or more are problematic? I'm an old dude just learning this stuff. Thanks for any help.

 

Are you talking about the individual coil packs on the AJ 16 engine?

My XJ-S (88.5) came with a primary and secondary ignition coil a distributor and an Ignition amp. My mechanic replaced both coils with a new coil that does the job that used to be done by two. That seemed to even out the idle.

If you have the AJ16 4litre 6, do you also have an ODB2 port? If so, maybe there is a log identifying a flaky coil pack? My '95 X300 has that engine and a data port under the dash on the driver's side.

Just a thought.

 

Thanks, this is really are great community of support and practical help. I'll relay these ideas to my (I hope capable) mechanic.

 

If it has a data port, you may be able to read the codes yourself with an inexpensive code reader. I bought a decent unit for not much money at Harbor Freight.

Since the 4.0 liter AJ16 is also in the X300 (XJ6 '95-'97), you might check that forum too.

Anyway, best of luck.