my vote is on vacuum leak. When the car idles down do you hear a hissing noise?
Idle Air Motor Problem.
Thread Starter
Pro
Joined: Jul 2005
Posts: 735
Likes: 0
From: Glasgow Va
Idle Air Motor Problem.
Hi I have been having some problems with the idle going up and down when sitting at a stop light.I put the scanner on it today.The idle air counts would be around 7 or 8 when it was idleing right.When the idle would speed up it would read any where from 55 to 75.I have cleaned the motor and the air ways.I also tried to reset it and it still does the same thing.Could anything else be causing this or does it just sound like a bad idle air motor.It has also stalled a few times when stopping also.
Thanks.
Thanks.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
The IAC does as it is told. The fact that the counts are changing, proves that it works. Now you have to track down why the ECM is overworking it. My best, first guess, is a vacuum leak somewhere changing the demand for air to maintain a smooth steady idle.
RACE ON!!!
RACE ON!!!
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
If you should find no vacume leak or anything obvious, my experience with those kinds of problems has been to change the motor (IAC). Our Firebird got 3 motors in about 10 years and the 98 Jeep got 1 so far. Sometimes cleaning works but seems not to last long for me. 
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
Originally Posted by pcolt94
If you should find no vacume leak or anything obvious, my experience with those kinds of problems has been to change the motor (IAC). Our Firebird got 3 motors in about 10 years and the 98 Jeep got 1 so far. Sometimes cleaning works but seems not to last long for me.
His problem is something else.. Map readings might indicate what exactly. Prob a vac leak as suggested.
-- Joe
Thread Starter
Pro
Joined: Jul 2005
Posts: 735
Likes: 0
From: Glasgow Va
I can't really hear any hissing sounds.I will go over all of the hoses good and see if I can find any leaks.The map readings range from 1.39 volts to 1.45 volts at idle and when it speeds up.TPS volts stays right at .58 volts.
Thanks.
Thanks.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
The change in MAP volts indicates a pressure change in the manifold, as might be caused by an intermittent vacuum leak. The steady TPS voltage only indicates that no one is playing with the throttle.
RACE ON!!!
RACE ON!!!
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Pro
Joined: Jul 2005
Posts: 735
Likes: 0
From: Glasgow Va
I have not been able to find any vacume leaks yet.This only starts to happen after the car has been run for a while.I still don't see why the ecm thinks it need to open the iac up more.The car is idleing fine until the iac opens up.I would think a vacume leak would make it try to close off the air not open up to let in more.I also noticed that when trying to merge on to the interstate with the ac on it starks to jerk and jump and kind of spit when its under a hard load.It will run 75-80 with no problems at all.I think my idle problem my be related to this problem as well but I am not sure.
Thanks.
Thanks.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
The IAC starts working as soon a the engine starts. A vacuum leak is unmeasured air that enters the intake that the ECM can't correct for. You might want to take the throttle body off and take the IAC out for a good cleaning. I consider this a maintenance item. When installing the IAC back into the TB, make sure the tip of the pintle is no further than 1 1/8" from the IAC flange. A new set of gaskets probably wouldn't hurt either.
Thread Starter
Pro
Joined: Jul 2005
Posts: 735
Likes: 0
From: Glasgow Va
Hi I noticed today that when I have the ac on it seems to idle fine.I know that will cause the iac to raise the idle some.It seems to idle smooth with no surging up and down.The whole engine just feels smoother with the ac on.I also though the 91 Z28 I had felt smoother with the ac on as well.Maybe it is just me.Anyone else think this.