Bad Idle IAC Issue?
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Le Mans Master
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From: Akron Ohio
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St. Jude Donor '09-'10-'11
Bad Idle IAC Issue?
I understand that the idle on a '90 is not adjustable, correct? When the car is cold and i start it i have to give it gas to stay running or it dies and it stumbles when i try to drive. Once its warm its ok. Would this be an IAC issue? How do I know if it just needs to be cleaned or if it needs to be replaced? If i replace it do I have to do anything like flash the computer or make it recognize the new one?
Le Mans Master
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From: South Carolina
St. Jude Donor '03 & '05
I understand that the idle on a '90 is not adjustable, correct? When the car is cold and i start it i have to give it gas to stay running or it dies and it stumbles when i try to drive. Once its warm its ok. Would this be an IAC issue? How do I know if it just needs to be cleaned or if it needs to be replaced? If i replace it do I have to do anything like flash the computer or make it recognize the new one?
Le Mans Master
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From: The Motor City
I understand that the idle on a '90 is not adjustable, correct? When the car is cold and i start it i have to give it gas to stay running or it dies and it stumbles when i try to drive. Once its warm its ok. Would this be an IAC issue? How do I know if it just needs to be cleaned or if it needs to be replaced? If i replace it do I have to do anything like flash the computer or make it recognize the new one?
Intermediate
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no trying to hijack this thread. but i'm dealing with a IAC issue on my 91. was planning on removing the TB as per what i've been told on here. my question is dealing with the coolant hoses running to it. i live in va, can i just bypass them?
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Le Mans Master
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Thanks guys for the responses. I wil try cleaning everything first and see how that goes.
Le Mans Master
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From: San Diego, Ca
Why L98 people have some compelling need to screw with Minimum Idle eludes me - doesn't seem to effect the LT guys, but maybe the screw is more difficult to access. Anyway, there's no reason for it get out of adjustment OR if it is, it's because someone decided it needed to be adjusted OR there's so much gunk crapping up airflow, you might as well dip the throttle body in a vat of acid.
As to your cold can't start/won't idle problem, a scan will tell you everything you need to know - is the Coolant Temp Sensor (which takes the place of a choke) accurate? If not, find out why and you'll solve your problem. Inoperative IAC motors/circuitry don't suddenly work better once the engine is warmed up.
vabatwho: I've never seen a documented performance gain by bypassing the coolant flow through the Throttle Body on a stock Vette. Nor will it miraculously cure some driveability issue. The basic IAC diagnostic test is this: Note idle rpm. Turn it off and restart it. Restart rpm should be higher but return to your noted speed within 60 seconds. If it does, there is nothing wrong with the IAC, it's circuitry or the ECM - you have some other issue.
As to your cold can't start/won't idle problem, a scan will tell you everything you need to know - is the Coolant Temp Sensor (which takes the place of a choke) accurate? If not, find out why and you'll solve your problem. Inoperative IAC motors/circuitry don't suddenly work better once the engine is warmed up.
vabatwho: I've never seen a documented performance gain by bypassing the coolant flow through the Throttle Body on a stock Vette. Nor will it miraculously cure some driveability issue. The basic IAC diagnostic test is this: Note idle rpm. Turn it off and restart it. Restart rpm should be higher but return to your noted speed within 60 seconds. If it does, there is nothing wrong with the IAC, it's circuitry or the ECM - you have some other issue.
Thread Starter
Le Mans Master
Joined: Oct 2007
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From: Akron Ohio
2025 Corvette of the Year Finalist - Modified
2024 C5 of the Year Winner - Modified
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St. Jude Donor '09-'10-'11
Why L98 people have some compelling need to screw with Minimum Idle eludes me - doesn't seem to effect the LT guys, but maybe the screw is more difficult to access. Anyway, there's no reason for it get out of adjustment OR if it is, it's because someone decided it needed to be adjusted OR there's so much gunk crapping up airflow, you might as well dip the throttle body in a vat of acid.
As to your cold can't start/won't idle problem, a scan will tell you everything you need to know - is the Coolant Temp Sensor (which takes the place of a choke) accurate? If not, find out why and you'll solve your problem. Inoperative IAC motors/circuitry don't suddenly work better once the engine is warmed up.
vabatwho: I've never seen a documented performance gain by bypassing the coolant flow through the Throttle Body on a stock Vette. Nor will it miraculously cure some driveability issue. The basic IAC diagnostic test is this: Note idle rpm. Turn it off and restart it. Restart rpm should be higher but return to your noted speed within 60 seconds. If it does, there is nothing wrong with the IAC, it's circuitry or the ECM - you have some other issue.
As to your cold can't start/won't idle problem, a scan will tell you everything you need to know - is the Coolant Temp Sensor (which takes the place of a choke) accurate? If not, find out why and you'll solve your problem. Inoperative IAC motors/circuitry don't suddenly work better once the engine is warmed up.
vabatwho: I've never seen a documented performance gain by bypassing the coolant flow through the Throttle Body on a stock Vette. Nor will it miraculously cure some driveability issue. The basic IAC diagnostic test is this: Note idle rpm. Turn it off and restart it. Restart rpm should be higher but return to your noted speed within 60 seconds. If it does, there is nothing wrong with the IAC, it's circuitry or the ECM - you have some other issue.
Thread Starter
Le Mans Master
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From: Akron Ohio
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St. Jude Donor '09-'10-'11
Ok. New news. I just took the throttle body off and the inside is covered in oil. It appears to be coming in from the pass side vacuum port that goes to the vent on the pass valve cover. What does this mean? Is there something I have to replace?
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Le Mans Master
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From: South Carolina
St. Jude Donor '03 & '05
Thats 4 starters/
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Le Mans Master
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I got the throttle body and IAC clean, everything was pretty dirty. When I pull the tube out of the valve cover there is nothing but a plastic elbow with the rubber hose going from it to the throttle body. I dont see any kind of PCV valve.
Le Mans Master
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From: South Carolina
St. Jude Donor '03 & '05
Wish I could help more. The LT1 tube going to valve cover, pass side, is for WOT use. Maybe you have mild blow-by and need an oil catch can to aleviate oil getting into intake.
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Le Mans Master
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From: Akron Ohio
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Le Mans Master
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From: The Motor City
Why L98 people have some compelling need to screw with Minimum Idle eludes me - doesn't seem to effect the LT guys, but maybe the screw is more difficult to access. Anyway, there's no reason for it get out of adjustment OR if it is, it's because someone decided it needed to be adjusted OR there's so much gunk crapping up airflow, you might as well dip the throttle body in a vat of acid.
Do not plug it, it is part of your PCV system. If you don't like it being able to pull from the pass. side valve cover, you would need a breather on the valve cover and then you would plug the TB. (keep the PCV valve).
Last edited by mseven; May 2, 2009 at 04:23 PM.
Racer
Joined: Jan 2007
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From: Muskogee OK
The hose provides clean air into the crankcase that is then pulled into the manifold, through the PCV valve on the drivers side cover. DO NOT plug the hose.
You can get a back flow at WOT, which is normal.
Pull your PVC valve and look for excess smoke or blowby if you're still concerned.
You can get a back flow at WOT, which is normal.
Pull your PVC valve and look for excess smoke or blowby if you're still concerned.
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Le Mans Master
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From: Akron Ohio
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St. Jude Donor '09-'10-'11
To address some things said the cap covering the idle screw is still on the TB. Also the car started normal and idles great. However I still have to look into the PCV valve because the oil in the TB concerns me. Thanks guys for the help.
Race Director
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St. Jude Donor '07
Replace the PCV valve.
Check your plugs while the engine is cold, say overnite, for oil wetness. You may have a leaky valve seal that is gumming them up, too. The oil will burn off when warmed up.
Check your plugs while the engine is cold, say overnite, for oil wetness. You may have a leaky valve seal that is gumming them up, too. The oil will burn off when warmed up.
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Le Mans Master
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As for the valve seals they were replaced less than a year ago when the cam went in.
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Le Mans Master
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It was a code 15. However I disconnected the battery for a bit then ran the car in the driveway and a code never showed up after about ten min.