Last week, my jeep started up and instantly was pegged to 2.5k to 3k rpm. Obviously not where a idling jeep should be... Jeep is a 1990 4.0 auto renix with a HO swapped in a few years ago. Done by using all the renix sensors, but using the HO idle stepper motor.

So here is what I have done, I ended up frying my tps somehow so I have replaced that and have it adjusted properly. I checked the map sensor and that wasn't showing the correct output voltage, the new one has been confirmed to be working. I checked the iat and it showed 1000k ohms at around 200* on the water gauge, but the ohms where continuing to drop as the engine was running. I'm assuming this is working somewhat properly. I haven't check the cts, but that is also used for idle speed.

What I do know is that with everything plugged in the jeep idles high, once I unplug the engine side of the TPS the idle drops to around 600-800, a normal range. If I unplug the idle stepper motor and replug the TPS in the engine revs up smoothly but then stumbles once the rpms dip, but then goes back to the smooth idle like before because the idle stepper isn't being used at this point. I have confirmed that the idle stepper does move and is what I believe is causing the high idle. Something is telling the computer that the idle needs to be increased but then removing the tps causes the pintle to move back out lowering the idle.

I'm searching for what is telling the idle stepper to retract causing a high idle condition. Any and all help would be appreciated.

 

Sounds like vacuum leak.

 

Im having a strange issue with my 98 as well. I just replaced the TPS and now it revs high at start up.

 

Go to www.cruiser54.com and do Tips 1,3,4,and 5.

 

I have got bad TPS from the big box retailers before, bad out of box and also bad in about a month, idled all over the place. mopar is best, jeeps very fussy about the engine management components.