Problems with idle (I think)
 

99 Jeep Cherokee Sport

Symptoms:
check engine light was on, and a check of the codes showed an issue with the idle air control valve
car would start ok but after driving even a short distance it would stall out when the engine slowed to approx. below 1200 rpm, i.e. when slowing down at any intersection or stop.
at times had extremely rough idle, i.e. the car was shaking

I replaced the idle air control valve. No change in the problems mentioned, and now the engine revs itself, going up to as much as 4000 rpm.

What next? After changing the idle air control valve is there a process I should go through to reset the idle air control system? The check engine light is still off. What else might be the problem?

Did you also check the wiring harness and connector? On a 20 year old vehicle wiring cracks, connectors get dirty or rusty, etc.

If the RPM's jumps up "on it's own", it means extra air is getting into the intake. Did you take care with the O-ring for the IAC?

I'd double check the IAC installation, double check the wiring and connector are sound, then look for other possible places where the air could be getting in. The intake/exhaust manifold bolts have a tendency to back out over time - not so much as they did on earlier years, but it's worth checking them. Cruiser says check them every 30k, so check them every 30k.

You should disconnect the battery completely and touch the terminals together for about 10 seconds, this will reset all the values the ECM has learned (unless you have a Fully functional scanner that can call for the ecu to reset all it's adaptives and base settings.).

I've had similiar issues that were traced back to a failed Throttle position sensor TPS.

Double check for any vacuum leaks or lines that may have possibly been damaged or dislodged during the replacement of the IAC

progress so far...
 

What I've done since my first post : removed the throttle body and cleaned it, checked the installation of the icv, made sure all bolts are tight

There seems to be a little progress but not much. The check engine light went out after I first installed the idle control valve. Now it's back on. I haven't been able to check the code yet. I also noticed that the tachometer needle stays at 1 most of the time but then will suddenly drop all the way down off the scale, sit there for awhile, and then go back to 1. Is this a clue? What component of the engine sends the reading to the tachometer? Perhaps that is where the problem lies.