1988 renix 4.0 5spd

Idles usually about 1100-1200rpms. In the 2-3months I've owned it only twice has it idled at 850-900rpms. But when I have the bi-level on and the electric fan kicks on the rpms drop to about 1k and stay solid.

While driving if I am coming to a stop and down shift or just toss it into neutral it sticks up around 2k at times and it takes a few throttle blips to get it to idle lower.

Where should I start? Any electrical tests I should start with, I know my grounds are good since the cables are new, the c101 has been cleaned of tar, and I've done crusiers54's ground cleaning.

 

Click on the link in my signature. Do posts #1,2,3, and 5. See what happens after that.

 

Our Idle Air Control is on the front of the TB. (We don't need to remove much to reach it). Also be gentle with the O ring. So besides the above there is this. Do be sure the "kickdown" cable isn't holding the throttle open...

Not sure to credit for this...I think they have a post 90.
CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality.

 

I just had the same issue with my 89. I would start it and it would shoot up to 3k. I would have to turn it on and off a few time before it would quit. I unhooked my IAC and it started normal and ran at 850. Replaced the valve and haven't had a problem since.