1988 Jeep Cherokee - Fast idle problem

Hi there, new to the forum....seeking some advice. I've got a 1988 Jeep Cherokee (automatic) (4.0L). When I start the vehicle, one of two things happens. It will either run at very low rpms (around 3-5 hundred rpm) and stumbles until it warms up, at which point it runs normal. Or, it will run extremely fast (around 2500-3000 rpm) and it does not slow down.

Has anyone had experience with this, or does any have any advice they can offer?

Thanks in advance.
Sharon

I was having simular trouble with my 89, combined with general rough running, especially when changing speed.

the biggest improvement was an easy fix. removed the 'idle speed stepper motor' (located on the throttle body to the passenger side of the TPS) and clean the plunger, mine was severly fouled and the 10 minute fix really smoothed out the running, especially at idle.

if that doesn't help, then check your sensors. your idle speed, timming, fuel mix are all controlled by the ECU, so odds are it's a sensor.

Thx for the advice, sorry didn't check the post sooner. Could you please detail what that part looks like (idle speed stepper motor) and where it is exactly? I am a total newbie when it comes to fixing things.

Thanks again for your help.

ok, I'll be real basic so just ignore the parts you already know.

start standing on the drivers side at the front of the jeep, looking towards the back of the jeep, when I give left and right directions it will be your right and left not the cars.

first find the throttle body (if you're not sure where that is follow the large black air hose from the aircleaner to the the intake manifold, the throttle body is between that black hose and the manifold.)

on the front of the throttle body there are 2 components with wires going to them, the one on your right is the Throttle positioning sensor (TPS), the one on your left is the idle speed stepper motor.

it's very easy to remove, just a couple of torx screws. I just sprayed about 1/4" of wd-40 into a baby food jar, and put the dirty end in to soak for 20 minutes then cleaned it up with an old toothbrush.

then it just bolts back up, and plugs in. there are no gaskets, or certain alignment to worry about.

JUST DONT PULL ON THE PLUNGER! I made the mistake of doing that trying to get the embedded gunk out and ruined mine. About 60 bucks to replace!

:banghead:

I had the same problem, same vehicle. A little cleaning went a long way.
-KojakLuvsDa'88Jeep

Would this solve my problem as well:

I have a 1990 with I6 and when I let it idle it constantly revs between 800 and 300, and doesnt matter if its warm, cold, it does it all the time.

yes, it will very likely fix it for you.

Would this solve my problem as well:

I have a 1990 with I6 and when I let it idle it constantly revs between 800 and 300, and doesnt matter if its warm, cold, it does it all the time.

there is a little srew in your in your trottle body that controls your idle.
this is located where your trans pressure cable and thotttle linkage connect to the thottle arm.the srew is what the trottle arm rest on.try turning that little srew to ajust the idle.

hope this is to some help.

I had exacly the same problem you are stating here that stumped several mechanics. Eventulay i figures out the problem was the throtal position sensor located just beside the automatic choke was geting stuck. Cost me like 89 bucks for the sensor at Autozone and the problem was solved.
Also make shrue you clean take off the large air intake hose and clean the area around the automatic choke plate as this gets pretty carbon scored when your tps frezzes up and will some times cause the plate it self to get stuck closed.
Thand

I had the same problem with my 1990, I had it hooked up to a diagnotic machine and it was the EGR Valve I'd try that if the throttle bod thing is clean.

what about my 87 jeep cherokee pioneer idles high at start up then sometimes will idle down after keep shutting off and starting back up

I would say it could be the EGR Valve but run it on a diagnostic machine to be sure.

This is a good step for a "high time" jeep. My 88 Laredo idled quite well actually, but while changing our the CPS, I checked the ISA, found it rather dirty and cleaned it. Rather take the time now when it's OK than deal with it when it's not.

I have an 89' Laredo that has been giving me grief. When I start it up it is idling just fine, but as the jeep runs the idle increases in increments. It takes about five to ten minutes for it to reach the highest point which is anywhere between 2000 and 2500 rpms.

check the tightness of the intake manifold bolts. the lower ones are hard to get to and require extensions and swivels. this is a known issue with the renix motors. the intake manifold bolts loosen up and let air in causing a vacuum leak which raises the idle.

not saying this will fix it but it is free and it is one possible cause! mike