IAC Question
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
IAC Question
OK, I've been fighting this hunting idle thing for too long. My question is: can I just disconnect the IAC, without causing any problems? I have replaced the IAC with a brand new one, made sure it was set to the proper length before installing, cleaned the throttlebody, and passages, and tried numerous times to set the minimum idle...each time I reset the minimum idle, it is ok for a while, then begins "hunting again. I ran my Datamaster, and the IAC showed Max IAC @ 160, and the IAC counts ranged from 0 to about 12....so I take that to mean the min. idle was just about right...I finally just gave up, and disconnected the IAC, and my idle is a rock steady 800...no up or down, just 800....car drives very well, idle comes back to 800 at a stop. Datamaster shows me IAC Max @ 160, and IAC counts range from 0 to 3...so....can I just leave the IAC disconnected, and drive it, or do I need to do something else
Cheers,
Michael
Cheers,
Michael
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 32
From: The Top of Utah
Of you disconnect the IAC, you will have no ECM controlled idle speed; no cold idle, no idle compensation for A/C on, etc. You have an idle problem, probably a vacuum leak, and the IAC makes it worse, because it is playing catch up, trying to stabilize the idle. Disconnecting the IAC is an improper hack repair, but it won't do any damage. It would be best if you fixed the real cause of the problem, rather than trying to band-aid the symptom.
RACE ON!!!
RACE ON!!!
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
Well, I've used EVERY method mentioned here on the forum....carb cleaner, propane, smoke, even tried an ultra sonic leak detector.....no leaks found. Gave up and took it to a well-known vette mechanic here in Houston to try and find any vacuum leaks.....no go, he could not find any either. So, I'm almost sure I have no vacuum leaks...that's what has been driving me crazy. Looking at Datamaster, when I log the car, the IAC counts never show to be over 12, yet the idle swings 2-300 rpm....just doesn't make any sense to me
Cheers,
Michael
Cheers,
Michael
Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
For diagnostic purposes only, disconnect the IAC to take it out of the equation. Then troubleshoot and see what results or answers you come up with. Don’t know how long it takes for your idle swing to occur, but you need that length of time +.
If you still have the idle problem, you can then keep looking for the solution. If it clears up and the idle is steady, then look toward the IAC, wiring, RPM feedback, computer and such for the answer.
If you still have the idle problem, you can then keep looking for the solution. If it clears up and the idle is steady, then look toward the IAC, wiring, RPM feedback, computer and such for the answer.
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
For diagnostic purposes only, disconnect the IAC to take it out of the equation. Then troubleshoot and see what results or answers you come up with. Don’t know how long it takes for your idle swing to occur, but you need that length of time +.
If you still have the idle problem, you can then keep looking for the solution. If it clears up and the idle is steady, then look toward the IAC, wiring, RPM feedback, computer and such for the answer.
If you still have the idle problem, you can then keep looking for the solution. If it clears up and the idle is steady, then look toward the IAC, wiring, RPM feedback, computer and such for the answer.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
7 Bolt-On Upgrades From Extreme Online Store to Level Up Your C6 Corvette
Pouria Savadkouei
How Likely Are These Five 2027 Corvette Rumors to Be True?
Brett Foote
9 Best Corvettes You Can Buy for Half Price (& 1 You Should NEVER Buy!)
Joe Kucinski
8 Very Best Corvettes of Amelia Island 2026
Joe Kucinski
Top 10 WORST Corvette Engineering Failures of All Time!
Joe Kucinski
10 Records the C8 Corvette Generation Has SMASHED (& 1 Glaring Failure)
Joe Kucinski
7 Wildest Corvette Concepts Ever Made
Brett Foote
Le Mans Master
Joined: Jun 2001
Posts: 6,694
Likes: 3
From: Less Talk, More WOT | Houston TX
When I disconnect the IAC, idle is rock steady @ 800....no up or down...used the FSM to check the IAC wiring, all checked out....pulled the IAC, and jumpered A&B on the ALDL, and the pintle moved as I would expect. Datamaster never shows any excessive IAC counts, they always stay between 0 and 12, even when the idle is swinging...tried 3 different ECMs in hopes it was a bad ecm, but the idle remained jumpy with all 3...I'm really frustrated with this!!
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
Well, I had the fuel rail off a while back, and I thought it would be a perfect time to check the injectors....left them connected, and kicked on the key....fuel rail pressured up, and not one injector leaked even a drop...left it that way for about 5 minutes, but no leaks....so unless something strange has happened, I don't think any of the injectors are leaking....
Cheers,
Michael
Cheers,
Michael
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 32
From: The Top of Utah
Originally Posted by mk842766
When I disconnect the IAC, idle is rock steady @ 800....no up or down.
RACE ON!!!
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
There is no question the IAC is causing the idle speed variation. The question is why. Another question is why doesn't the IAC changes show up in the data log? Maybe the answers to both are related. Possibly a minor closing of the IAC raises the vacuum just enough to trigger a small, intermittent vacuum leak. Have you closed down the IAC and checked and/or tried to set the minimum air adjustment? If you persist, you can conquer it.
RACE ON!!!
RACE ON!!!
Cheers,
Michael
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 32
From: The Top of Utah
Once warmed tp, it only takes a minute of over 40 MPH driving to "set" (reaccumlate) the IAC. "There is no question the IAC is causing the idle speed variation. The question is why. Another question is why doesn't the IAC changes show up in the data log? Maybe the answers to both are related. Possibly a minor closing of the IAC raises the vacuum just enough to trigger a small, intermittent vacuum leak."
RACE ON!!!
RACE ON!!!
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
I agree, something else I see is:
Is this a stock motor? 800 would be pretty high idle rpm. to set min air at operating temp. for a stock motor I would be looking at around 500-550rpm for min air setting (IAC disconnected)
Questions, What does it say for learned iac steps at that point ? It goes down to 0? when this is going on, what else do you see in the scan, big timing advance/retard changes ? blm moving?
It seems to me that the IAC would be moving trying to compensate for idle swing, also going down to 0 sounds to me like it cannot compensate enough. Are there any possible vac. leaks?
Is this a stock motor? 800 would be pretty high idle rpm. to set min air at operating temp. for a stock motor I would be looking at around 500-550rpm for min air setting (IAC disconnected)
It seems to me that the IAC would be moving trying to compensate for idle swing, also going down to 0 sounds to me like it cannot compensate enough. Are there any possible vac. leaks?
Last edited by mseven; Jun 19, 2007 at 09:37 PM.
Melting Slicks
Joined: Aug 2003
Posts: 3,374
Likes: 385
From: Lakeville MI
0-12 counts is only enough air to cause about a 75 rpm swing.
Roughly 0.06 gm/count * 12 counts = 0.72 gm/sec.
Roughly 100 rpm/1 gm/sec * 0.72 gm/sec = 72 rpm.
If you're getting a 300 rpm swing, there is more to the story than just the IAC. I'm sure varying spark advance in this rpm region is a big part of the problem.
I would work on the spark table to improve idle stability in the desired idle rpm region.
Open loop fuel vs load can also contribute to idle surges, but it does not have as big an impact as the spark advance.
Roughly 0.06 gm/count * 12 counts = 0.72 gm/sec.
Roughly 100 rpm/1 gm/sec * 0.72 gm/sec = 72 rpm.
If you're getting a 300 rpm swing, there is more to the story than just the IAC. I'm sure varying spark advance in this rpm region is a big part of the problem.
I would work on the spark table to improve idle stability in the desired idle rpm region.
Open loop fuel vs load can also contribute to idle surges, but it does not have as big an impact as the spark advance.
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
Originally Posted by mk842766
When I disconnect the IAC, idle is rock steady @ 800....!
---------------------------------------------------------
Cam specs?
check in the table idle rpm vs. coolant temp.
I expect you have a value OVER 800 rpm at operating temp.
If not, this can be a problem and your 0 IAC steps are the result.
Adjust this table ONLY if the cam REQUIRE an higher idle.
Check also the CTS in front of the manifold.
Check also the O2 xconts. If you have Long tube headers and NOT a heated O2 sensor, this cause an intermittent rich condition.
Do not trust too much to to Datamaster on IAC counts...
on my 90 with datamaster I can see the IAC steps varying EVEN with the iAC connector disconnected.
-Beppe-
When I disconnect the IAC, idle is rock steady @ 800....!
---------------------------------------------------------
Cam specs?
check in the table idle rpm vs. coolant temp.
I expect you have a value OVER 800 rpm at operating temp.
If not, this can be a problem and your 0 IAC steps are the result.
Adjust this table ONLY if the cam REQUIRE an higher idle.
Check also the CTS in front of the manifold.
Check also the O2 xconts. If you have Long tube headers and NOT a heated O2 sensor, this cause an intermittent rich condition.
Do not trust too much to to Datamaster on IAC counts...
on my 90 with datamaster I can see the IAC steps varying EVEN with the iAC connector disconnected.
-Beppe-
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
I agree, something else I see is:
Is this a stock motor? 800 would be pretty high idle rpm. to set min air at operating temp. for a stock motor I would be looking at around 500-550rpm for min air setting (IAC disconnected)
Questions, What does it say for learned iac steps at that point ? It goes down to 0? when this is going on, what else do you see in the scan, big timing advance/retard changes ? blm moving?
It seems to me that the IAC would be moving trying to compensate for idle swing, also going down to 0 sounds to me like it cannot compensate enough. Are there any possible vac. leaks?
Is this a stock motor? 800 would be pretty high idle rpm. to set min air at operating temp. for a stock motor I would be looking at around 500-550rpm for min air setting (IAC disconnected)
Questions, What does it say for learned iac steps at that point ? It goes down to 0? when this is going on, what else do you see in the scan, big timing advance/retard changes ? blm moving?
It seems to me that the IAC would be moving trying to compensate for idle swing, also going down to 0 sounds to me like it cannot compensate enough. Are there any possible vac. leaks?
Cheers,
Michael
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
0-12 counts is only enough air to cause about a 75 rpm swing.
Roughly 0.06 gm/count * 12 counts = 0.72 gm/sec.
Roughly 100 rpm/1 gm/sec * 0.72 gm/sec = 72 rpm.
If you're getting a 300 rpm swing, there is more to the story than just the IAC. I'm sure varying spark advance in this rpm region is a big part of the problem.
I would work on the spark table to improve idle stability in the desired idle rpm region.
Open loop fuel vs load can also contribute to idle surges, but it does not have as big an impact as the spark advance.
Roughly 0.06 gm/count * 12 counts = 0.72 gm/sec.
Roughly 100 rpm/1 gm/sec * 0.72 gm/sec = 72 rpm.
If you're getting a 300 rpm swing, there is more to the story than just the IAC. I'm sure varying spark advance in this rpm region is a big part of the problem.
I would work on the spark table to improve idle stability in the desired idle rpm region.
Open loop fuel vs load can also contribute to idle surges, but it does not have as big an impact as the spark advance.
Cheers,
Michael
Thread Starter
Pro
Joined: Aug 2000
Posts: 666
Likes: 0
From: Richmond TX
Originally Posted by mk842766
When I disconnect the IAC, idle is rock steady @ 800....!
---------------------------------------------------------
Cam specs?
check in the table idle rpm vs. coolant temp.
I expect you have a value OVER 800 rpm at operating temp.
If not, this can be a problem and your 0 IAC steps are the result.
Adjust this table ONLY if the cam REQUIRE an higher idle.
Check also the CTS in front of the manifold.
Check also the O2 xconts. If you have Long tube headers and NOT a heated O2 sensor, this cause an intermittent rich condition.
Do not trust too much to to Datamaster on IAC counts...
on my 90 with datamaster I can see the IAC steps varying EVEN with the iAC connector disconnected.
-Beppe-
When I disconnect the IAC, idle is rock steady @ 800....!
---------------------------------------------------------
Cam specs?
check in the table idle rpm vs. coolant temp.
I expect you have a value OVER 800 rpm at operating temp.
If not, this can be a problem and your 0 IAC steps are the result.
Adjust this table ONLY if the cam REQUIRE an higher idle.
Check also the CTS in front of the manifold.
Check also the O2 xconts. If you have Long tube headers and NOT a heated O2 sensor, this cause an intermittent rich condition.
Do not trust too much to to Datamaster on IAC counts...
on my 90 with datamaster I can see the IAC steps varying EVEN with the iAC connector disconnected.
-Beppe-
I agree with you on the IAC steps in Datamaster...on my scans, with the IAC disconnected, it still shows that my counts vary from 0 to 3...less than with the IAC connected, but I would expect them to remain at 0....
Cheers,
Michael
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
Beppe, stock cam...at operating temp, with IAC disconnected, idle remains rock solid 800...CTS checks out ok by the FSM, and in Datamaster, the temp looks correct...I do have headers, and a heated O2...cross counts look ok.
I agree with you on the IAC steps in Datamaster...on my scans, with the IAC disconnected, it still shows that my counts vary from 0 to 3...less than with the IAC connected, but I would expect them to remain at 0....
Cheers,
Michael
I agree with you on the IAC steps in Datamaster...on my scans, with the IAC disconnected, it still shows that my counts vary from 0 to 3...less than with the IAC connected, but I would expect them to remain at 0....
Cheers,
Michael
close a bit the throttle blades via TB screw... just to lower to 650-700 rpm without the help of the IAC.
do reset, TPS 0.54 ...bla blabla..bla..
MAINTAIN in the chip the 850rpm idle, with just the purpose to use MORE IAC speps than 12.
I know that ALL are saying that around 20 stps is good. IMO and i my experience a level of 30 or 40 or 50 or in the middle of 0-160 is good.
I don't like to stay too much near the border line (near 160 or near 0).
...Just another suggestio to try...