Iac
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5th Gear
Joined: Apr 2013
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Iac
I have read most of the info on IAC adjustment! I have an '85 that I have completely gone thru, but am still having an IAC issue. When I start the engine from cold, it does not idle high and then slowly return to a normal idle speed. It starts fine, but wavers up and down. sometimes appearing as it may stall (but it doesn't) and then when hot, the idle is too high. typically 670-750 and it still wavers 20-30 rpms as it idles?
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Joined: Aug 2002
Posts: 3,842
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From: SANTA CRUZ CA
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ECM calibrates the IAC every time you cycle the ignition. There is no need to "set" it to any specify length. I just push mine all the way in before screwing it in. Turn ignition on, turn ignition off, done.
Do a minimum idle adjustment:
http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
Do a minimum idle adjustment:
http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
Safety Car
Joined: Aug 2009
Posts: 3,751
Likes: 276
From: Melbourne, Victoria, Australia
The ECM calibrates the IAC every time you cycle the ignition. There is no need to "set" it to any specify length. I just push mine all the way in before screwing it in. Turn ignition on, turn ignition off, done.
Do a minimum idle adjustment:
http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
Do a minimum idle adjustment:
http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
If the idle adjust screw has been adjusted in the past it will not start easily hot or cold, when you set your IAC or TPS you need to disconnect the battery or ECM connector near the battery.
When the base idle has been set correctly, she will start easy hot or cold and have a high idle at 1200 rpm and slowly drop to 600rpm.
It will hunt a little till the ECM has re learned how to manage your engine, that usually takes half hour or so of driving and gets better every day.
Good luck
Le Mans Master
Joined: Aug 2007
Posts: 8,735
Likes: 412
From: WV
with my 383 my tuner had me set the IAC counts to 40, and my idle to 750 rpms which is way different then stock...Don't forget, every time you reset your IAC your also changing your TPS setting..If your idle is hunting up and down you may want to check for a vacuum leak.....WW
Last edited by WW7; Apr 29, 2013 at 08:58 PM.
Team Owner
Joined: Jul 2004
Posts: 28,467
Likes: 3,288
From: Hartford WI
IIRC, on a 90-91 the TPS is read as closed reading when you startup the car.
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Joined: Aug 2009
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From: Melbourne, Victoria, Australia
If the base idle has not been set to spec the ECM won't have a hope of controlling the engine correctly, it needs to be done because the ECM expects it is correct.
Maybe if your engine is wildly modified you need a modified chip or piggyback ECM for the engine.
Team Owner
Joined: Jul 2004
Posts: 28,467
Likes: 3,288
From: Hartford WI
I have a 383 and it is supercharged the std ECM and chip run it all, and I still did the base idle setup and she runs great easy start hot of cold and smooth stable idle.
If the base idle has not been set to spec the ECM won't have a hope of controlling the engine correctly, it needs to be done because the ECM expects it is correct.
Maybe if your engine is wildly modified you need a modified chip or piggyback ECM for the engine.
If the base idle has not been set to spec the ECM won't have a hope of controlling the engine correctly, it needs to be done because the ECM expects it is correct.
Maybe if your engine is wildly modified you need a modified chip or piggyback ECM for the engine.
When I set the IAC, I make sure the timing is spot on and there are no air leaks, etc, etc. Shut down everything but the engine and let the ECM adjust it the way it wants. If the idle counts are higher than say my target of 30, I would turn the screw in, rev it, let it settle down and recheck the counts. When it is lower, I would turn the screw out and do the same. Once I hit the target IAC counts, I shut down, start it up and go.
I wouldn't say I am wildly modified till I had a blower like you or juice. However, I screwed up and made my compression too high so when it gets rebuilt, it will be lower than 10.8 and will accommodate a blower.