Hi guys,

My recently acquired 96 XJ is almost road ready but there's one problem I can't seem to figure out. Occasionally after coming to a complete stop the RPMs will drop to around 200 before they bounce back up to the normal 800-1000 range. I'd say this happens about every 1 out of 5 stops. Once it drops, the engine sounds like she's going to stale but immediately bounces back and runs as if nothing happened. The idle is good otherwise and is not rough at all. This will only happen once per stop. I could pull in my driveway, come to a complete stop, let the idle drop to 200, then let it run for 20 minutes and it won't do it again until I raise the RPMs, let them drop back to idle, and about 20% of the time it will drop to about 200 before bouncing back again.

It has a 4.0L.

Symptoms list:
* Doesn't do it until I start moving
* The drop doesn't always happen immediately after coming to a stop. It may take 30 seconds to a minute
* As far as I can tell, while driving there's no signs of the engine wanting to cut out
* Engine temp doesn't seem to matter. It could do it immediately after pulling out of the driveway and coming to the stop at the end of my street or after 20 minutes of driving
* If it does it at all, it's only once per stop/idle. It won't happen again until I start moving and come to another stop.
* No check engine light

I immediately thought this was the IAC. I pulled it and cleaned it up (it was pretty dirty) but it hasn't made any difference.

Here's the list of things I've replaced/repaired since picking her up:
* Both O2 sensors
* AC compressor (it had a bad clutch; I really thought this was what was causing it as immediately after the drop would occur the bearings in the clutch would start making a ton of noise). The new compressor has made no difference.
* New plugs (copper autolites gapped to .035), wires, dist cap, and rotor
* New belt
* Oil change
* Removed IAC and cleaned it up with throttle body cleaner
* New air filter
* Checked for broken vacuum lines; only found one for the heater valve that I've since fixed but I will check again

Any ideas? I'm stumped.

 

Have I stumped the XJ gurus?

I'm going to pull the throttle body and clean it. Hopefully that will resolve the issue. I'll update when I get a chance to do it. Probably won't have time until week

 

In case anyone comes across this thread I fixed the problem. After removing the throttle body it became clear the PO replaced the TPS. When doing so, one of the two mounting screws was presumably lost as only one was holding the sensor in place. To compensate for only the missing screw, someone way over tightened the one screw and drove it through the sensor as it was cracked pretty badly. The o-ring was also missing although there seems to be some debate whether or not one is actually needed on the TPS.

I replaced the sensor with a BWD that I picked up at Advance for $35. The Mopar branded one is $113 at my local dealer. I've read this is one of those parts where you should go OEM but hey, I saved $78 and the aftermarket one seems to work alright. No more RPM drops

 

nice good call, had a feeling it was going to be resolved by cleaning the throttle, just not expecting the tps cracks. thats the problem with POs can never be trusted.

 

amen. unfortunately with both of my xj's, i've learned this the hard way

 

Fortunately (or maybe unfortunately) for me i knew the PO through my work(Auto Tech). I had been doing all the mechanical repairs and inspecting the XJ for nearly 3 years before i made an offer on it. and thats after i knew he overheated it twice water pump leaked out/ heater valve burst. ran it out of oil, oil pressure switch. but god damn if that engine sounds smooth and runs great. something about the XJ just makes ya love it.

 

Thanks for posting a great symptom description and especially coming back with the resolution!

I have the EXACT same symptoms, performed an IAC and throttle body clean, and still had the issue.

I was planning on testing the TPS, but don't have an analog multimeter to see the voltage response.

I'm gonna gamble and go grab a new TPS without testing If it's not the solution, then I'll have an OEM spare.

 

if you're buying mopar, i believe it comes with a new rubber o-ring. if buying a different brand make sure you use an o-ring on the tps where it mounts to the throttle body. i don't know for sure if it matters but the factory put one on.

good luck!

 

Reporting back that a OEM TPS has fixed my symptoms described in the OP.

It also temporarily fixed a shifting issue I have (AX-15). The issue is a quick decrease in engine RPM when pushing the clutch in to shift. I usually have to add some throttle when shifting to make a smooth&quick shift.

For about a week, the new TPS made engine rpms drop more slowly for a smooth shift. However, I am back to a quick decrease now. I am going to look at my IAC again.