OK, I have an 87 cherokee 4.0L. My idle is at around 1200ish right now. Everyone has been telling me to clean or Replace the IAC. I replaced it, and cleaned the WHOLE throttle body. No change. I also checked the MAT sensor, i had an extra so switched it in, no change. Here is what it does. I start the car, it starts fine reving to 1700-2000 and immidiatly droping to 700ish, then it slowly revs up to about 1200 and holds it, when i put it in gear, the tach drops to zero and right away and comes back up to about 500 as if it almost stalled out. I dont get it, i checked for vaccume leaks, replaced the IAC, i donot have a scan tool to check voltages etc. Sometimes i wonder if my jeep will ever work right..... :-(

I dont know what is wrong with your jeep but my jeep had a similar problem wherre it would have a high idle even in gear every once in a while. but that all stopped when i fixed my fuel injector leak and replaced the o rings and one fuel injector.

I dont know if these two are connected or if it is a fluke but i think that might have fixed it for me ( Knock On Wood) it might be that the smoke like gas mixture affected one of the sensors in the engine.