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Idle Air Control Valve- High Idle

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#21 ·
Nibbley15 said:
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?
Click to expand...
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.
 
Discussion starter · #22 ·
Alfons said:
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.
Click to expand...
There wasn't any loose manifold bolts. I tried pulling the vacuum lines and I have great vacuum, not to mention when I yanked off one of them the idle shot up even higher, so I'm going to stab a guess and say I really don't think it's a vacuum leak.
Going to chase after the temp sensor next.
What is the normal vacuum pressure?
 
Discussion starter · #24 ·
brentd711 said:
See if this is any help

4.0L Computerized Engine Control (pdf file)
Click to expand...
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:
Image


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
 
#25 · (Edited)
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
 
#27 ·
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 :)
 
#30 ·
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.
 
Discussion starter · #31 ·
Que89YJ said:
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.
Click to expand...
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?
 
#32 ·
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.
 
#33 ·
Nibbley15 said:
Mine does the opposite, cold idle is anywhere from 700-900 warm is 1400-1700
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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?
 
Discussion starter · #34 ·
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.
 
Discussion starter · #35 ·
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....
Image

Image

Image
 
#36 ·
Pull a warm engine compression test to check prior to a valve job. The white discoloring on the valve stems is really no indicator. The valves make their seal around their face where the flanged end meets the valve seat in the head. As long as this is good, you're golden!

Max

Sent from my HTC Incredible Droid
 
Discussion starter · #37 ·
Mean Max said:
Pull a warm engine compression test to check prior to a valve job. The white discoloring on the valve stems is really no indicator. The valves make their seal around their face where the flanged end meets the valve seat in the head. As long as this is good, you're golden!

Max

Sent from my HTC Incredible Droid
Click to expand...
Sweet, I'm just used to the old way of thinking- white means lean burn; however it appears that most of these 6 cyl that it is just fine, when they are black it appears to be bad. Thanks!
 
#38 ·
I'm sure your intake leak induced a lean burn condition resulting in the whitish color. But I would look for more definitive proof of damage than just the color.

Maybe pull a compression test every so often just for peace of mind. Even if you get a lower reading on one or more cylinder, I still wouldn't assume valves until a cylinder leak test indicated specifically that the valves leak.

Max

Sent from my HTC Incredible Droid
 
Discussion starter · #39 ·
I'll run a warm compression check and a wet one probably next week to see what's up. Waiting on a phone call from a buddy with a welder so I can get that last little crack touched up on the exhaust header.
Thanks again Max.
 
#40 ·
Good deal, sounds like you've got it pretty well handled!

Max

Sent from my HTC Incredible Droid
 
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Que89YJ
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