oldgraymule

I replaced my idle air control (IAC) valve today.
My 1989 4.0, 5 speed manual trans, 205,000 mile Cherokee has been idleing around 1000 RPM's for the past few months.
I removed the old IAC and cleaned it with carb cleaner and air. I reinstalled it and it idled at about 750-800 RPMs on the factory installed dash tach. When I test drove it, the idle returned to 1000 RPM. So I figured that the IAC was the problem.
I purchased a new one and installed it as per the instructions.
They said to bring the engine up to normal operating temperature and then drive it at 40 MPH, which I did. At that speed, the ECM will command the IAC to reset, and proper idle should result.
I started the engine after installing the new IAC and it idled at about 750-800RPMs. When it got to normal operating temperature, I took it for a test drive hitting 40 MPH and more. The idle again returned to 1000 RPMs.
Does anyone know what is going on?
What is telling the ECM to set the idle at 1000 RPMs?
Thanks for the advice.

jeeperxj91

I always thought cars would tend to idle a little higher in colder weather.

MontanaXJer

Check out this thread: https://www.cherokeeforum.com/f51/helpful-renix-links-1397/ for lots of useful information on your Renix era Jeep. Pay particular attention to the link for "The ReniX files". I highly suggest reading that one all the way through. It has a ton of great info on high idle causes for the 87-90 4.0L engines.

What else have you replaced and or tested?

BowtieGuy3

Anyone else notice this is a double post?

oldgraymule

I found a post from BlueXJ on this problem I am having.
I disconnected the battery terminals and touched them together for 20 seconds. Then I did 12 restarts over a few hours letting it idle for a few minutes each time. Each time I started the engine I didn't touch the accelerator pedal, basicly I just turned the key on, then a few minutes later I turned the key off. The idle speed was normal each time.
Then today I let it warm up and then took it for a ride reaching speeds of 50 MPH.
When I came to a stop the idle returned to the 1000 RPMs.

I took it home and checked the TPS.
Pin A showed 4.91 volts.
Pin C showed .82 volts.
This was within tolerances from a RENIX engine sensor diagnostics site.
My Jeep shop manual gives different voltages, so I don't know which way is correct.

ulendo

dont forget the obvious either - make sure theres some slack in the throttle cable, and tranny detente cable, and that someone, at sometime hast simply screwed in the idle speed screw too far...

having said that ( and guessing you checked it already if you;re running diagnostics on the TPS, and IAC) if you have the shop manual / any service manual for the renix era stuff, look up the exhaust back pressure valve. its an auxilliary control circuit to the EGR system, and if its toast ( and they often are) can/will hold the egr valve open, and result in a high idle. pretty easy to test it - remove, and plug the egr vacuum line, and make sure the pintle is moving freely enough to close completely. ( you can see the EGR pintle if you look straight down from the top) Another symptom of EBPV failure is that it'll somtimes correct, and idle too low/stall.

if its like mine was, someone tried to 'fix' the EBPV problem by turning up the idle screw to make it always idle at 900-1000 rpm, so dont be too surprised if you find a number of things out of spec.

oldgraymule

I will check the EGR system later this week. I didn't think of that.
I ordered this Jeep new, so I know its history.
Thanks