My 98' Cherokee sport( 4wd, manual trans., 4.0L, 142k miles) has the strangest idle symptoms.
When I start the car when the engine is cold, it kicks right over. It idles like normal for about 45secs, then at about 800rpm, the engine goes into this stumbling idle. It does this rough idle for about 4mins, and then once i give it the throttle, it runs fine. If i try to drive the car before the 4min mark, it stumbles down the street like it's lacking power or starving for gas. The exhaust smells like raw fuel during the 4 min warm up stage. Once the car is warmed up, it drives normal and never reoccurs until I let the engine get cold again.
Here's my dilemma: I have had 2 different mechanics look at it and run diagnostics and functional Tests.
We have cleaned the throttle body, and IAC. Checked fuel injectors for leaks, checked plugs, coils, and sensors. Fuel Pump is good. CPS was replaced 6months ago. TPS registers good on Diagnostics.
Mechanics went on to:
Then replace the IAC motor, Coolant temp. Sensor, Intake air temp sensor, and distributor plate. It still does the same thing as before.
I'm really at a lose.... Someone please help us before i end up going broke dumping parts into it for no reason and paying mechanics.
It's your "upstream" O2 sensor. They have a heating element that triggers during cold start warmup and adjusts fuel accordingly. When the element does not function properly, the computer is fooled into thinking the engine is warm and runs rich as if it were. There's a technical term for the routine.....closed loop or something, I just don't recall. O2 sensors are relatively inexpensive and the problem you are decribing is just like the one I had on my '98 until I changed that sensor. Good Luck!