High Idle

[jon1998]

I just bought my first Jeep, it's a 1989 Comanche 2wd with the 4.0 liter inline-six. Among its many issues is a strange idle. When I start the Jeep cold, its revs to 2000rpm then settles to 1500rpm. The idle speed fluctuates up and down but eventually creeps its way to 2000rpm. Another perhaps related problem is that after driving around for a while the engine will seem to cause the jeep to shake very badly. It also seems to run pretty rich most of the time. I have already adjusted the TPS, cleaned the butterfly valve in the throttle body, replaced the IAC valve, the MAP sensor, and the coolant temperature sensor (the one on the side of the engine block near the driver's side motor mount). None of this has worked. Any suggestions anyone might have as to what could be causing these issues and what I should do to fix this would be greatly appreciated. 

[HOrnbrod]

Sounds like you might not be in closed loop after warmup. Try a new O2 sensor.