Need some help with my nieces XJ. It is a 1999 Sport, 4.0, automatic. Unfortunately I can only relate what she is telling me, but I have run out of ideas and have no way to go check it out myself. She says it rev's up on its own (not sure how high it revs) when sitting at a stop light and will overheat (just found out it only heated up twice, not sure how hot tho).

After doing some reading here I suggested:
1) clean IAC
2) Clean throttle body and valve
3) test TPS
4) check for vacuum leaks around intake

and this is the response I got from her boyfriend:
Replaced tps sensor, ignition coil, spark plugs, wires, cap, rotor, fuel filter and pump, ecm, cleaned out throttle body and checked all fuel injectors. Checked for evap leaks around intake, no leaks. It has a surging issue with the coil and injectors. It's an inline 6, 4.0, 2wd with 118,000 on it. Started following wiring going from ignition coil to harness found two broken wires shorting to ground, repaired wiring, problem still persists. The engine will go up in rpms at idle and while running. Very random and not a constant pattern. After flashing the new ecm and installing it, the problem was fixed for a day and a half and then came back. I did some research on it and found out the '97-'01 4.0L Cherokees had a lot of problems with the harness going to the ecm in the engine compartment. I believe it's the harness, from the electrical tests I did and all of the parts we changed out. I also traced all visible wiring to search for anymore broken wiring or any metal to metal contact and nothing else was found. Any advice would be great at this point, the only open recalls I found from the dealer was the air bag light coming on and gauges all dropping (clock spring) nothing regarding any wiring.

Sorry about all of the second hand info, but it is all I have. If there is anything at all you can think of please let me know and I will pass it along. If not, no worries, figured it was worth a shot asking.

Have a good one and as always Thanks!
Dro

 

I'm sure the experts will chime in, but my only advice is to concentrate on finding the overheating problem first. If that has the 0331 head, you risk cracking it with it overheating, but from what I've read, not many 99's have those heads. Could be something as simple as the fan clutch, but IMO that should get remedied first. Good luck.

 

Overheating can have a negative effect on how a motor runs. All the parts that were replaced will have zero effect on overheating. The XJ cooling system is not an area to take lightly.

 

IAC was suggested, but not reported as been cleaned...

 

The overheating was reported by my niece, so not sure how hot it actually got and not sure what she considers "overheating".

Will mention the IAC again.

 

Basically, overheating is overheating. No such thing as a little overheating or a lot of overheating. Too hot is too hot. Might want to spend some more time with the niece getting some more details. Again, the condition of the XJ cooling system is not to be taken lightly.

 

Thanks for the help folks, her father the "mechanic" finally got off his ***** and decided to help.