I've been dealing with a high idle for a few months and can not figure it out. It is a 1998 cherokee 4.0. She idles at about 1500rpm with its old IAC in. I bought a orielly's one and it jumped it up to nearly 3k, so I returned that and assumed it was junk. So this time I bought a MOPAR IAC and it sends my RPMS up to around 2500. The pintle does NOT move in the old one or the new one. and the old one is definitely extended out further. I tried resetting the ECU but still acts the same.
Any help here would be appreciated.

 

Did a little more testing. tested the voltage at the harness and 3/4 pins read 0, the 2nd one read 1.3. So I am assuming I am not getting power to it. I am going to try a spare ECM out of a parts vehicle to see if that works before tracing the wires.
These are the only ideas I have

 

vacuum leak

 

IAC is a stepper motor so there won't be a signal there unless the ECU is commanding it to move. Sounds like yours isn't if the idle runs high with a new valve (with it's pintle pulled in) and lower with the old valve. The ECU should correct their position within a few moments.
You could check wiring continuity from the plug back to the four IAC Drivers in the PCM. Drawing:



.

 

^^^^^^ Test for this.

 

My 1998 XJ does the same thing. I would really like to know what you came up with. Please get back to me ASAP.