Ready to give up..... ridculously high idle 89 Cherokee

[89 Jeep Cherokee]

I purchased what I thought was going to be a cheap tow vehicle about 2 years ago. It is a 1989 Jeep Cherokee 4X4 with a (4.0 Litre) fuel-injected, in-line, 6 cylinder engine with 187,000 miles or so on the odometer. After replacing both upper and lower ball joints on both drvr and psgr sides, (2) new front hubs, (1) new front brake hose, both window regulators, a complete major tune-up and installing (4) new tires ALL since my purchase 2 years ago. I have a yet another problem!.......When the vehicle sits overnight or, until when it is completely cooled off....... when you crank the vehicle to start it, it will hold a constant idle somewhere between 2300-3200 rpms for anywhere from 10-25 minutes non-stop (the exhaust system gets so hot from the revving....it actually made the padding under the carpet smolder to the point of nearly catching the entire vehicle on fire last week)! If you listen closely, you can hear what sounds like a slight miss or sputtering sound after it revs for about 15-20 minutes just before the idle returns to it's normal position at about 900rpms or so.....

I have had this to (2) different mechanics work on it and have had both the throttle position sensor and air control valve sensor replaced and neither has solved the problem.....IF ANYONE OUT THERE HAS HAD THIS PROBLEM AND HAS BEEN ABLE TO SOLVE IT......CAN YOU TELL ME WHAT YOU OR YOUR MECHANIC DID TO CURE THE PROBLEM??? I WOULD BE MOST APPRECIATIVE....THANK YOU IN ADVANCE!

 

[SteelHeadz]

Check it for a Vacuum leak around the gaskets.

 

[Bounty__Hunter]

Vacuum leak would make sense, small enough that part expansion after warm-up seals the leak.

 

[FREDMCGUIRE]

I had the same problem on my '88 Comanche 4.0 L and the easy fix is to pull the wire connector from the Idle Speed Control (which is attached to the throttle body by two small torx screws, is on the front left of the throttle body and has a lever that sets the idle speed). If it sticks due to carbon build up and runs a high speed, which is a common problem, it can be cleaned by taking it apart and soaking in throttle body cleaner or the like, or by installing a new one. By pulling the connector off, the control doesn't work and doesn't set a high idle but in cold weather it takes a 2 to 3 minute warm up before driveability is OK (kind of like driving a car without a choke - I have a CJ-7 with that problem).
Good luck and let us know how things work out.

 

[MoparMatt77]

I have used Mopar combustion chamber cleaner p# 4318001 to some success. 2 TBIs when sensors were gumed up. Used Mopar CCC and then took TBI off and cleaned it from bottom. Fixed idle problem, passed cleaner with emmissions test too. Jeep 2.5L and 4.0L love to collect carbon build up.