1992 idle speed problem
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Intermediate
Joined: Nov 2004
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From: Huntsville Al.
1992 idle speed problem
My '92 LT1 has an issue with idle speed and I need some opinions on how to proceed from where I am. The car had three distinct idle speeds, 1200rpm, 750rpm and 450rpm, all with trans in park. After talking to some local "gurus" It was suggested I clean the IAC pintle valve and seat, and clean the throttle body. I cleaned everthing up, and ended up replacing the IAC. I have only run the car for a short time, but it now only idles at the higher speed (around 1200-1300rpm in park, obviously too high). Does the computer have a "learn" function? (my Lumina has that function) or should I look for something else? I've been all over the engine, and am sure there are no vacuum leaks, with the old IAC, during the times it idled at 750rpm in park, it would idle about 550rpm in drive and ran so smoothly you could barely tell it was running..that makes me feel like there wouldn't be a vacuum leak. Anyone have any suggestions?
Team Owner
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From: Fort Knox, KY
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If you've cleaned the TB and IAC motor, I'd suggest looking for vacuum leaks.
You can take it to a shop that has a "smoke machine" that puffs smoke over the engine and determines leaking locations.
You can take it to a shop that has a "smoke machine" that puffs smoke over the engine and determines leaking locations.
Le Mans Master
Joined: Feb 2006
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From: Louisiana
Sounds like it could be a vacuum leak, but these are usually associated with the idle surging. When you installed the IAC did you make sure the pintle was no further that 1 1/8" from the flange and did you make sure you checked the connection as well? It would help to verify this with a scan tool.
Burning Brakes
Joined: Nov 2005
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From: 1992 LT1 6 speed Cayman Islands
Originally Posted by RRT vette
Sounds like it could be a vacuum leak, but these are usually associated with the idle surging. When you installed the IAC did you make sure the pintle was no further that 1 1/8" from the flange and did you make sure you checked the connection as well? It would help to verify this with a scan tool.
You MUST adjust the new IAC valve before installing it per the FSM
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
With this kind of problem (with all my cars) after I cleaned it and made no progress, I just like to replace the IAC motor. They sometimes just go bad and you can spend much time messing around and until you change it you will never know. I have never had much luck cleaning them and having the fix last, I like new ones. 
Thread Starter
Intermediate
Joined: Nov 2004
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From: Huntsville Al.
Problem solved
Hi everyone,
Thanks for all the replies! I did adjust the pintel length of the new valve to the specified length of 1.125" when I cleaned everything. After telling an experienced GM tech what was going on, he asked me if the car had been driven for a while with the bad IAC unit in it. I said yes, as far as I know. He then told me I needed to disconnect the battery for a short period of time to allow the copmputer to clear itself (I had forgotten that I had to do the same thing to my Lumina when it was having some sensors replaced). After pulling the negative cable off for a couple of minutes, put it back on and the car is doing just fine...have driven about 50 miles so far, and it is idling just where it should be. I am one happy camper right now, hope it keeps it up!
Thanks again,
JB
Thanks for all the replies! I did adjust the pintel length of the new valve to the specified length of 1.125" when I cleaned everything. After telling an experienced GM tech what was going on, he asked me if the car had been driven for a while with the bad IAC unit in it. I said yes, as far as I know. He then told me I needed to disconnect the battery for a short period of time to allow the copmputer to clear itself (I had forgotten that I had to do the same thing to my Lumina when it was having some sensors replaced). After pulling the negative cable off for a couple of minutes, put it back on and the car is doing just fine...have driven about 50 miles so far, and it is idling just where it should be. I am one happy camper right now, hope it keeps it up!
Thanks again,
JB
