DESERTXJ206

IV got a 88 Cherokee 4.0 with an intermittent idle problem. IV removed and cleaned the throttle body and the IAC motor plunger. It still Continues to do it. Its got all stock parts, have not replaced anything yet. When I start the Jeep sometimes the idle jumps very high~ no tach so I will assume around 2500-3000k. Sometime if I pump the break it will go down to normal. Currently its getting worse. It will randomly start to idle high when driving or sitting at a light. Prior to this I noticed when decelerating sometimes it would act up downshifting. Like basicly staying at a high idle until almost completly stopped then downshift and calm down to normal idle. I have not started testing anything because I figured I'd ask here first. I will test the Tps tonight with cruiser54s write up and see where that gets me.

freegdr

possible egr valve sticking causing vacuum leak which causes high idle...

cruiser54

Possible bad TPS. Got a voltmeter and know how to use it?

Possible loose intake manifold bolts. Got a 9/16 wrench?

DESERTXJ206

Yeah cruiser you walked me through some steps with my 90 and it turned out to br previous owner messed with idle set screw. Ill be checking the Tps tonight. Also going to replaceme the o ring on the iac.


Could that be the issue with the trans not downshifting in time.

cruiser54

TPS could be. Got my instructions on testing BOTH sides of your TPS?

DESERTXJ206

I have never tested the auto trans side of the plug. I just know how to do the other one from the last time you helped.

cruiser54

It's included in my write-up. Still got it or need it again?

DESERTXJ206

Ok cruiser help me out here, I replaced my Tps with a spare I had(not 100% sure if spare is good). My refrebce voltage is 4.73. So 4.73 x.83 is 3.9259 volts. I set it to 3.92 volts on the meter. I also replaced the main vacuum harness from valve cover and intake side with a new part. Everything's clipped together tight, Tps is tight, iac is tight and visually inspected it move when throttle plate opened up.

Now every other start I get is super high 3000+ rpms. Idk where to look next. Sometimes it will start to idle high then catch itself and go back down. Sometimes it will just make the 4.0 scream. What to do and where to look would be great help.

je4reyk

Ok my 88 dose that ,next time it runs away un-plug the IAC, and it should idle down to normal, I just left it unplugged ,but any way you need to check your grounds at the dip stick ,then check the temp switch on the side of the block,that one tells the computer if the engine is cold

cruiser54

You're checking the TRANS side of the TPS , not the ENGINE side. That side could be good and well adjusted but the engine side is not. There are 2 completely different "wipers" inside the TPS and they work opposite of each other. One for trans (square connector) and one for engine (flat connector). Re-read the instructions and set your TPS using the flat connector.

Backprobing the flat connector:

Key on, measure reference voltage between A and B.
Key on, measure output voltage between B and C.

Output voltage should be 17% of reference voltage.

The ENGINE side setting of the TPS seems more critical. Let the TRANS end up wherever it does. I only use the Trans side for diagnostics of trans shifting issues.

je4reyk

What I suspect is happening is the computer sees a cold engine and tries to compensate by opening the IAC motor ,just like the choke system on a carbed engine. If the computer gets an intermittent signal that the motor is not warm then it might do this.

cruiser54

[QUOTE=je4reyk;1822926]What I suspect is happening is the computer sees a cold engine and tries to compensate by opening the IAC motor ,just like the choke system on a carbed engine. If the computer gets an intermittent signal that the motor is not warm then it might do this.

Except for the fact that someone has screwed with a factory adjustment and it needs to be brought back to stock, and the TPS was adjusted incorrectly. Those problems need to be addressed first. Also, a CTS will cause a rich running condition before it ever causes a high idle condition, by design.

je4reyk

TheT.P.S dose need to be corrected,but that should not be a random near red-line racing engine the IAC is some how opening up at the wrong time,if the cts causes a rich mix then the engine is going to idle fast

freegdr

something not addressed yet how would pumping brake affect tps guys?

cruiser54

Redline is 4800 RPM plus. FYI, I have experienced numerous faulty TPSs and the symptoms described are the signature symptoms.

The IAC fully open will not raise the idle to 2500 RPM.

Another FYI, the CTS does not cause the idle to go up. Have you ever noticed that a Renix only flares up a bit on a cold start and then idles it's normal speed immediately? It doesn't work like a choke.

Fregdr. I have seen where holding the brake pedal down under load with a bad TPS will get it to come down. I don't know why.

The point I'm trying to get across, with much resistance and guessing, is THE THROTTLE BUTTERFLY SCREW HAS BEEN MESSED WITH AND NEEDS TO BE BROUGHT BACK TO STOCK AND THE TPS ADJUSTED PROPERLY BEFORE GOINg OFF ON OTHER TANGENTS. Only makes sense to eliminate known variables before proceeding.