I drive a 1999 jeep xj 4.0. It seems to have a slight rough idle both when cold and warm (drive and accelerates just fine). I've replaced the destributor cap, plugs, cables, and idle control valve over the last few months. All that has helped but it still seems a little rough to me. Also, the RPMs at idle fluctuate slightly, up and down by no more that 20-30 RPM. Is a little vibration at idle to be expected? I want to know if I'm being OCD! Thanks!

 

That fluctuation is normal. It's a 20+ year old car so don't expect it to be super tight. With OCD, an xj might not be the best car to own hehe.

I've been chasing a similar thing over the last few years. A little stumble at idle, small but noticeable.
Did all the things, including the harmonic balancer and injectors. I could keep going with it for ever because there would be so many possible things contributing.

​​​​​​The list is almost endless from anythingwitj the ignition system to fuel delivery. Timing is not so much adjustable with these xj years.

Only tuneup item I missed there was the rotor.
It might be good to check the TPS as well as simple measure, after you cleaned the throttle body ( especially the plate). Relatively simple and doesn't require new parts.

 

I upgraded to 4 hole injectors and the idle in my 98 smoothed out a little. Still not the smoothest.

Its just how alot of 4.0s are.

Side note
Replacing a distributor cap but not the rotor is like replacing a brake caliper but not the pads

 

Might be a small vacuum leak. They can leak at the throttle shaft. Worth doing some hunting.



https://www.youtube.com/results?sear...tle+shaft+leak



https://www.youtube.com/results?sear...+a+vacuum+leak