Can't figure out my idle problem
Thread Starter
Melting Slicks
Joined: Mar 2008
Posts: 2,351
Likes: 222
From: Delta B.C. Canada
2023 C7 of the Year Finalist - Unmodified
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Can't figure out my idle problem
92 LT1: Overall she runs great, but.........sometimes after restarting when it's at operating temperature, the idle sits at 1500-1800 RPM for a minute or two, and then drops to where it should be (600-700). Sometimes it will suddenly idle at 1000RPM in gear and then after a few minutes go back to normal. I've removed and cleaned the TB including the IAC orifice, and just put in a new IAC. I'm pretty sure it's not a vac leak, because I would expect the problem to happen every time and not just randomly.
Does anyone have any suggestions as to what to look for?
Does anyone have any suggestions as to what to look for?
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
92 LT1: Overall she runs great, but.........sometimes after restarting when it's at operating temperature, the idle sits at 1500-1800 RPM for a minute or two, and then drops to where it should be (600-700). Sometimes it will suddenly idle at 1000RPM in gear and then after a few minutes go back to normal. I've removed and cleaned the TB including the IAC orifice, and just put in a new IAC. I'm pretty sure it's not a vac leak, because I would expect the problem to happen every time and not just randomly.
Does anyone have any suggestions as to what to look for?
Does anyone have any suggestions as to what to look for?
Thread Starter
Melting Slicks
Joined: Mar 2008
Posts: 2,351
Likes: 222
From: Delta B.C. Canada
2023 C7 of the Year Finalist - Unmodified
2019 C4 of Year Finalist (appearance mods)
2015 C4 of Year Finalist
Been away for a few days........
No, the idle is normally totally smooth. No wandering or roughness at all. Check Engine never comes on either.
No, the idle is normally totally smooth. No wandering or roughness at all. Check Engine never comes on either.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 207
From: Orlando FL
I would get a scanner and see what the counts are doing. See if RPM follows the counts being programmed. The IAC is driven by the ECM and very possible the ECM is bad regardless of what the counts say. I would also check the respective pin connections between the IAC and ECM.
Drifting
Joined: May 2002
Posts: 1,562
Likes: 1
From: "Drive like Hell, you'll get there faster." Tucson AZ
92 LT1: Overall she runs great, but.........sometimes after restarting when it's at operating temperature, the idle sits at 1500-1800 RPM for a minute or two, and then drops to where it should be (600-700). Sometimes it will suddenly idle at 1000RPM in gear and then after a few minutes go back to normal. I've removed and cleaned the TB including the IAC orifice, and just put in a new IAC. I'm pretty sure it's not a vac leak, because I would expect the problem to happen every time and not just randomly.
Does anyone have any suggestions as to what to look for?
Does anyone have any suggestions as to what to look for?
If the LT1 has one too, have your TPS(Throttle Position Sensor) checked. Not many mention it up here, but mine actually went apesh1t on me in my '90 some time back and caused idles in that range with strange sudden shifts.
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Drifting
Joined: Nov 1999
Posts: 1,502
Likes: 12
From: Yorktown Heights NY
You put in a new IAC and if I remember correctly you have to have the ECM re-learn the new IAC, This can only be done with the original GM scanner. I know because I bought one specifically for this purpose 
Thread Starter
Melting Slicks
Joined: Mar 2008
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From: Delta B.C. Canada
2023 C7 of the Year Finalist - Unmodified
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Dang. I've only got Volume 1 of the FSM, and 6E is in volume 2. Is there something more to be done besides the reset procedure?
Le Mans Master
Joined: Jan 2006
Posts: 5,621
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From: Orlando FL
This is true provided the ECM is working correctly.
Le Mans Master
Joined: Dec 2004
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From: Myrtle Beach SC
St. Jude Donor '05-'06
I agree there is really no reset procedure where a tool is required. If you want to start from square 1, disconnect the battery and that will reset the ECM. The ECM monitors the RPM and adjusts the counts of the IAC for a targeted RPM given those conditions from external inputs (engine temp, outside temp, A/C, etc) to the ECM. The ECM will find the correct setting (IAC counts) for condictions, for a pre determined RPM setting (this is the learning procedure).
This is true provided the ECM is working correctly.
This is true provided the ECM is working correctly.
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
You can read codes without a scanner from the ALDL connector under the dash, above the gas pedal. The search function on this forum will tell you how. If you get a code, post it and I will post the page in the manual that has the diagnostic tree.