I recently bought a 90 model cherokee renix laredo with 150k miles. everything is fine except for the idle. when it first starts it runs fine at around 7-800 rpms then after about 3 minutes of warming up it goes up from there, it'll jump to 1000 then all the way to 1800 then back to 1500 fluctuating, then after it reaches operating temp it wont leave about 1700 rpms, it sucks driving down the road trying to stop the thing. i've replaced the tps and the IAC. The Tps cured my shifting problem, wouldn't shift out of first gear unless i took the fuse out and shifted manually. What else should i be looking at?

 

Check the vacuum hose from the map sensor to the side of the throttle body... mine cracked an had the same symptoms.

 

i already checked vacuum hoses and put new lines everywhere, i'll check again

 

check you 02 sensor, i was having issues like yours and i chased the prob around for half a year. i bought the sensor about 7 months ago , just never put it in cause i was told it wasn't the prob. finally changed it anyway and what a diff. way more power and nice smooth idle. the sensor was toasted(sounded like a worn out baby rattle when i shook it) my 88 has 162 k miles on it and had the original o2 sensor.

 

i also had the same issue on my 90 wound up being fixed when my ecm was replaced allways a possibity

 

Clogged EGR valve could be another potential problem as well. EGRs are a bad issue in the RENIX system. Even the Haynes manual mentions that the EGR sucks.


Have you tried running seafoam through the system? It may not cure your problem but might make it better. I had a small revving when cold issue, it would just stall out. Ran seafoam through the system and it completely fixed it.
I used to have a high idle, near or at 1500rpm, and replacing the EGR fixed it for the most part at least. Cold idles at 700rpm, but slowly creeps up to 1000rpm.