2001 4.0... So I've had a bit of a rough idle since I bought the Jeep in June. The main symptom is while I'm idling RPMs will jump up and then fall back down, I'd describe it as a little surge like 100-200 RPMs every couple seconds. Feels like when the AC compressor turns on but it's not the compressor. I'm usually not too worried about this but as Spring approaches I do want to get a plan of attack for working it out.

What I've done...
Replaced IAC
Cleaned Throttle Body
Temporarily replaced TPS... I also did the voltage tests on the TPS to assure the old one was still working
Fuel Pressure Test... I tested this early on and all looked good. I've since replaced the rail and no longer have a Schrader valve to test =[
Replaced Fuel Injectors w/ 4-hole
Sprayed for vacuum leaks. (I sprayed around the intake/exhaust manifolds and didn't get a reaction)

Next things on my mind are coil pack and O2 sensor. So now you know why I've been putting it off...

Are there any other tests I can run before throwing parts at it? Am I missing something?

Thanks!

 

Good luck with this! I could never figure out a similar issue my 01 had. Mine wouldnt jump rpm but instead felt like it would do a single misfire every couple seconds. I replaced absolutely everything on the engine, including rebuilding the engine (thanks to a cracked head) and could not fix the issue. The only thing I didnt replace was the crank and cam sensors. I even went as far as throwing on new pre cats, all 3 o2 sensors and the fuel pump and still no change. It never set the check engine light off so I just kept driving it and never had any issues.

Not related but somewhat related, my Evo did the same thing too. It ended up being an exhaust leak. The gasket between my down pipe and cat was blown. Its located about 3ft away from the main o2 sensor so I didnt think it was an issue. Sure enough, looking at my wideband when it was idling, the afr was going lean, then normal, then lean then normal causing the rpm to go all over the place every couple seconds. Simple gasket swap made everything go back to normal.

My Jeep did have a small exhaust leak at the header collector that I never fixed so it might have been the issue the whole time.

 

Good to know.... mine has been doing virtually that same exact thing the past couple months. It's just a slight change, but seems to run fine. And I've done the same checks that you have too... with the same negative results, so ruling out each of those one at a time.
My XJ did not come from a rust belt area and is virtually rust free...but I wouldn't be surprised if there's not a happy little pinhole somewhere along the exhaust or blown gasket that could be causing it too. Might check into that in the Springtime.

 

Thanks! Sounds like it took you through the ringer! I've always hesitated calling it a miss, but it is indeed what it feels like. I would expect a code tho... I too assume my header is cracked. One would think since the exhaust crack is so common this issue would be more prominent when discussing the topic. I look forward to others comments... I had an exhaust leak at the same gasket you mention and had it cut-n-weld last fall, thus making a future header swap a bit more involved but not out of the question. I think I mucked it up when I replaced the head. As I like to relate those exhaust bolts are by far the rustiest thing I've encountered on my XJ. Cheers!

 

I agree... that's exactly what I've kind of thought on mine too... Almost feels like a very slight miss. But after replacing several things that would be the most common culprit of that and it's there... the exhaust could very well be the issue.

 

I guess you guys could try unplugging the top 2 o2 sensors (1 for 99 and down), let it idle and see what it does. That way there should be no way the ecu can correct the afr since it will have no idea what its actually doing. Not the most simple test (for pre cat vehicles) since there is no room under the intake but at least its free!

 

I like it! I'll give it a go. If I remember correctly those connections are not much of a pain