I have a 1988 Cherokee 4.0L with 300,000 Miles on it. As of recently, whenever I start it up, from cold or warmed up, it will shoot up to 2000-3000. I can stop it and start it again and after a few tries It always comes back down. Sometimes it will be totally fine and not happen at all. Once it’s running right, it is totally fine for the rest of the drive.

I don’t believe it to be a vacuum leak due to the inconsistency of it, and when the IAC is unplugged (stuck at previous idle speed), the issue goes away, meaning the IAC is letting in the air.
I have replaced the IAC twice.
I have replaced the TPS very recently and adjusted it correctly, replaced it again.
I have checked grounds.
Cleaned C101 connector
Checked throttle cable
Wiggled everything

Anyone know what could be going on? I feel like i’ve tried everything and I’m sick of this.

 

RENIX is very sensitive to poor sensor grounds. You say you've checked the grounds, on the surface that sounds like a visual check, if not I apologize. The best way to check the grounds is with a multimeter, back-probing between the "B" terminal of the TPS connector and the negative terminal of the battery. You are looking for less than 1 Ohm of resistance (steps two and three of the attached link). If you find that your grounds are good, then the thing that worked for me was to locate an NOS MOPAR TPS, which can be quite spendy, and install that in place of the aftermarket TPS. Renix TPS Adjustment | cruiser54

 

Did you clean your throttle body including the IAC cavity?

What brand sensors did you buy?

You can get the FSM and the Renix manual here: https://cruiser54.com/?page_id=365

 

Yes, i’ve cleaned my throttle body. And I tested my grounds with an ohmeter, all reading only a few ohms.

TPS is duralast

 

I sometimes had trouble like this on my '96. if you have a '98 I doubt you have a Renix system, probably the OE Chrysler computer.

I traced the TPS signal back to the 3 pcm / computer connections and made sure I had proper grounds and good, clean +5 volt Vref and clean signals to and from the PCM on my jeep. Giving everything a good testing, and cleaning of the terminals from the PCM can help.

Also, I had to get 2 IACs from Napa, as one operated backward when installed., I swapped to the other part number they listed and things operated correctly.

 

It’s an ‘88, has the remix system for sure. I’ll try cleaning ECU Connector.

Ive already replaced the IAC twice, first with a super cheap questionable one, second with a reputable brand. I still have slight doubts about it though, do you think it’s worth trying for a third time?

 

Oops, somehow I read 98, not 88.

Sorry about that.