Lately as the weather warms up my cherokee's idle has been jumpy. Sometimes down around 300-500rpm to 1500-2000rpm or sitting at 1000rpm in park. I have cleaned and replaced the iac with a good used one off my buddy's old motor. Still does the same thing. What could be the causes of this problem. I just want it to idle normal again.

 

Clean your grounds yet?

 

Intake manifold bolts. Make sure they are tight. Common problem on the older 4.0s.

 

Last tune up ?

 

Did a tune up when I bought her last year. Intake is tight. I've cleaned, replaced and added extra grounds. I've cleaned all the connectors in the engine bay and what I can get to under the dash.

 

No missing just eratic idle correct...

 

Low power and MPGs? If so, it's probably the EGR, either the valve or solenoid.

 

TPS is probably on it's way out. Does it ever idle real high upon starting?

Ever do the sensor ground test?

Cruiser’s Renix Sensor Ground Test
　

This sensor ground circuit affects the CTS, TPS, IAT, MAP, ECU and diagnostic connector grounds. It’s very important and not something to overlook in diagnosing your Renix Jeep as it is common for the harnesses to have poor crimps causing poor grounds. If any or all of the sensors do not have a good ground, the signal the ECU receives from these sensors is inaccurate.

Set your meter to measure Ohms. Be sure the key is in the OFF position. Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.

Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.

You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.

Revised 06/12/2012

 

I have adjusted the tps and it did smooth out idle for a few months. It doesn't miss and it runs lean when idle gets stuck high. Still revs up when starting an idles down but most of the time it never goes below 1000

 

Good possibility someone messed with the throttle stop screw. Here's how to check it.


Cruiser’s Renix Throttle Body Butterfly Adjustment

Okay. Let's start from scratch. First off, that's not an idle adjustment screw. It's a throttle butterfly stop screw. It's purpose is to allow the butterfly to be as close to completely closed as it can be without binding or wearing into the throttle body. It was never intended to be adjusted in the field. But, Uncle Bob didn’t know that, did he?

Engine off. Back off the butterfly stop screw with a 3/32" allen wrench until the butterfly is completely closed. Now. turn the screw in until the FAINTEST movement of the butterfly opening is detected. This can be done more easily with the throttle body removed. If you remove the throttle body, be sure to replace the gasket underneath it after thoroughly cleaning the old one off.

Revised 07/07/2012

 

Let me clarify things. Most of the time it won't idle below 1000 but sometimes it will idle around 300 and it acts like it stalls when letting off the throttle to make a stop. When it idles around 300-500 which was normal when I bought it when switching the ac on the idle would go up to 800rpm.

 

Not to thread-jack but, every now and then when I start my '88 Cherokee, I get a high idle that wont come down. I've noticed it only does it on a warm start and sometimes I have to shut it off and restart it few times to idle normal. sound like grounds or TPS problem?

 

My buddy with an 88 also had that same problem. His gets very bad on warm starts. Mine does it almost all the time.

 

Here's some photos from my jeeps idling. First off it my 88 comanche idling.



Ac switched on with compressor engaged




Now my 89 cherokee
Normal idle



Ac switched on with compressor engaged




Comanche has 213710 miles
Cherokee has 194227miles
Comanche has a newer head that I put on myself.

 

Mine is starting to do it more often, possibly due to the hotter weather, I dunno.. But I should probably due that ground test.
For yours, have you searched for vacuum lines that may be kinked or anything? maybe thats why sometimes its high, sometimes low.