Find your vacuum leak. It sounds like you have a leak and the IAC is trying to compensate for it, then when you drive, it re learns what is normal and then you get the high idle. It doesnt take a large leake to make the idle go up.

Check all of your vacuum lines and connections. If you cant find anything obvious, you can spray carb cleaner around the vacuum lines with the engine running, listening for a change in idle speed. When you hear it, look in that area very closely for a split line, dry rot, etc.

 

Def sounds like a vacuum leak.

 

If the manifolds or the throttle body have been removed/reinstalled, that woiuld be the first place I would look - around the gaskets. With a leak that size, you should be able to hear some level of "hiss" that isn't attributed to the air rushing into the throttle body.

 

Make sure you don't have a sticky throttle plate. It will do the same thing. When I had that happen to me , I shot some CRC in it and worked it loose. Any carb cleaner would also work. There's a lot of contaminates in this ethanol we're forced to buy now! The stuff is crap!

 

Tried the vac leak/carb cleaner look around, there was no change in idle;
tonight when I get a chance I'm going to remove a vac line one by one and plug it (thinking maybe the 4WD lines?)
Sticky throttle plate? That's a new one, did spray cleaner down the throttle body didn't do anything but make the engine bog down.
Thanks for those that replied.

 

Would an exhaust leak manifold to down pipe cause this?

 

I am having the same exact problem. I thought I got a bad IAC from Quadratec, so I got one from Auto Zone and another from the stealership, only to have the same result. I use a propane tank to look for vacuum leaks and found none. I have been taking the IAC out and turning the plunger out at a half turn each time. Over the coarse to five days I have been able to get idle down a little but not where it needs to be. Engine at normal operating temp, idle is still around 1300-1400. Re-adjusted throttle cable, no change.

 

If the manifolds or the throttle body have been removed/reinstalled, that woiuld be the first place I would look - around the gaskets. With a leak that size, you should be able to hear some level of "hiss" that isn't attributed to the air rushing into the throttle body.

I had loose bolts at the header/exhaust manifold. Check them manually to be sure.

 

Checked the manifold, no leaks, I can only find an exhaust leak between the header and the downpipe (just before the O2 sensor). I was curious if the O2 sensor can play with the idle? Has anyone seen this type of behavior before from an exhaust leak?

 

The type of leak that you're describing does have an effect on the oxygen sensor, but that would make the engine run rich, not fast idle. You mentioned that you had some loose manifold bolts and tightened them - the fact that they were loose, could mean that the intake is still leaking to some degree (you don't need much of a leak to raise the rpm). You used a cleaner to spray for your vacuum leak test and didn't find anything - not all cleaners are volatile enough to be useful in this test, I've had better luck using propane, but again this test doesn't always work (the smaller the leak, the less chance you'll have of catching it with this test), so I tend to plug off all vacuum lines at the manifold with the exception of those that are needed to run the engine (MAP, fuel pressure regulator, and that's about it) and add a vacuum gauge to one vacuum port to see what reading I get. I rely on the vacuum gauge readings for a lot of problem diagnosis.

 

I had a high idle (3000-4000 RPM) and it turned out to be a leak in the intake manifold gasket.

Spraying some carb cleaner around the intake manifold where it meets the head would tell you for sure. You said you tried the carb cleaner. Did you try it around the intake manifold?

 

Yep, been there done that. Thanks though.

 

Bump,
Anyone, know if an exhaust leak between the header and downpipe can cause a hi-idle?

 

Nibbley, I don't believe the leak in that area will cause the high idle, because I don't have the leak, but do have the idle. I have gotten my idle down to 1100 rpm, but after a couple of days the IAC making the automatic adjustments the gets too low that my Heep wants to stall. Starting to think that I would have better luck if I went to a junk yard and pulled an IAC from there, because the three that I have bought are different from the original.

 

Nibbley, I don't believe the leak in that area will cause the high idle, because I don't have the leak, but do have the idle. I have gotten my idle down to 1100 rpm, but after a couple of days the IAC making the automatic adjustments the gets too low that my Heep wants to stall. Starting to the that I would have better luck if I went to a junk yard and pulled an IAC from there, because the three that I have bough are different from the original.

Thanks! I'm still chasing this one down,
I feel we are on the right track here, it has to be something that works with the IAC that tells it what to do (from another sensor?)

EDIT: I have the orginal sensor and the new sensor and they both do the same thing, first start idle great, after driving idle goes up.

 

Exhaust leak will not cause a high idle.

 

I have a caliper gauge and have the measurement down to 2 13/16 for the total length of the IAC. If i could keep that measurement then great, because idle is about 1300rpm cold and 900prm warm. The problem is the auto adjustment keeps moving (over time) the plunger out til the rpm's drop to low.

 

Mine does the opposite, cold idle is anywhere from 700-900 warm is 1400-1700

 

This statement is still bugging me. My issue has stabilized somewhat after setting the total length of the IAC to 2 13/16, complete cleaning of the IAC housing and installing a new MAT. Right now cold idle is 1400-1700rpm and warm is 700-900. My issue is everything gets reset after I turn the key off. If I start back up immediately, the idle is at 1400-1700, dropping after a minute or so. Being that you situation is opposite I can't help but wonder if a couple of wires were switched, telling the ECU that your engine is cold when it is not, but if this were the case then one of the three controllers (MAP,temp,TPS) should throw a code. Have you had a chance to test the MAP yet?

 

Im going to take a shot in the dark here. Maybe the ECU is thinking cold its in cold idle. Try checking the temp sensor by the thermostat. Did you clean out the throttle body and the IAC air passage when you had it out?

 

I just thought of that thermostat just this morning, need to get in the books to see if it's reading right. I cleaned out the air passage about a week or so ago.

I adjusted the IAC a bunch last night, taking measurements etc. I now have it adjusted to a cold start @ around 800-900 RPM's; warmed up it floats around 1100-1300; getting closer, but still working opposite so I'm still going to check the thermostat (computer side), since the gage is operating good.

 

See if this is any help

4.0L Computerized Engine Control (pdf file)

 

Thanks! Was looking for that chart.

OK, so here might be a silly question, and trust me I know wiring so yes I feel dumb asking this,
I am getting an infinity reading for Ohm's from the coolant temp sensor. I tried 2 different meters (digital), 2 different sets of wires. Checked the ground and had hardly any resistance, so I know the meter was working.
Tried checking the resistance between the two contacts on the sensor, tried one side to ground, then the other side to ground, nothing, nada, zilch. Please advise did I do this wrong? I am testing this plug:

EDIT: Had the meter set to Ohm's not K Ohm's since it was a cool motor had to have in the thousands to pick up a reading, LOL; tested against a known good one and had the same exact readings. Around 6,000 ohm's

 

I just ordered one of those today for my 2.5L ($25 @ NAPA). I also had infinity when I checked ohms so I unplugged the sensor and it runs just the same. My problem is a wicked ragged idle on cold weather starts (no high idle).

eta...my '90 2.5L CTS is in the intake manifold

 

OK, so the temp sensor is good to go, and everything is the same.... Now what?

EDIT: brentd711, $16.99 @ autozone

 

No listing at all at Autozone for a 1990. The '93 and the '90 are completely different.

I found the source of a "no-start when engine hot" by unplugging every sensor I could find, one at a time, and trying to start it. Process of elimination and it's cheaper than putting all new sensors in

 

I had the same problem about a week ago. I took the IAC like you did and cleaned it really good. My problem went away. I suggest checking the Throttle Postion Sensor.

 

I've had a TPS go bad before, different symptoms.

 

Nibbley, advance comes down to 3 sensors: Map (Same as vacuum advance on the old distributors, Temp sensor (Cold and warm idle), and the tps.

 

I know that is correct, however an out of limit TPS, and MAP will drop a code, and all I get is code 55. My shift light comes on (MAP), and when I push the cluth while arriving to a stop the RPM's are around 1500 ish, when I finally arrive to a stop the RPM's drop down to 1100-1300 ish (depending on it's mood). Also when I pull a vacuum line the RPM's go up, plug it and goes back down. This weekend I'm going to pull the exhaust donut out and replace it (take out known bad things). Don't think this will cure it, however maybe the O2 sensor is thinking rich/lean condition and more or less gas causing the high-idle?

 

No, a bad sensor doesnt fail out of range always. There in lies the dig with OBD1 over OBD2. Obd1 is a fixed range with OBD2 looks at rate change and range. You need to measure the TPS and Map to make sure they are ranging correctly.

 

Just got the exhaust leak fixed today, replaced the donut on the header manifold to down pipe, found that I had a cracked header/manifold too. Got that welded, runs smoother. Currently after that change here is my latest: sensors are all working as advertised, from the tests I can perform. No codes from the computer, the O2 sensor was pretty white (not normal goldish color).
Now the idle cold is around 800-900; warm is anywhere from 1100-1300 depending on it's mood. Next I'm going to pull the battery cable, to reset the computer and see what she does. She still has plenty of power, was able to maintain 55 @ 1450 RPM's in 5th on a flat road on 33's.
To answer the question on wires switched, it's stock wiring, nothing looks spliced together, I haven't pulled the dash out, but from what I can see with a flashlight, looks good under there.

 

After driving around on this for a week after reseting the computer, same stuff. Finally found the other exhaust leak (far right of the exhaust manifold) by #6 cylinder. So I decided to finally pull the exhaust header so I can replace the gasket. After pulling the intake & exhaust out, found out my gasket was delaminated bad and leaking by #6 on the intake & exhaust side; so IMHO this was probably causing my high idle. Now to get some feedback, my intake was coated with crap from the injector holes all the way to the valves, no big deal I guess, looks like someone was pouring something in there (seafoam?). Anyway here is a pic of the exhaust valves, they are white; so please let me know am I close to a valve job here? Or can I get at least a year out of these before I go and do a head job on this? Will this even hurt anything? Please advise, thanks in advance....