WOT vs. IAC
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
WOT vs. IAC
HAPPY NEW YEAR!!!
'96 LT4, ZF6, Cam, headers, 58MM TB, starts and runs great.. New development... last couple of times, after running thru the gears 1-2-3 at WOT the motor will not return to normal idle (approx. 900RPM) but hangs at 1200RPM. I can blip the throttle several times (and the TB blades seem to close completely as the linkage on the TB solidly hits the stop screw), with no change in the high idle so I don't believe this is a mechanical issue. If I turn off and restart inmmediately, the Idle returns to normal on restart. I've R&R'd and cleaned the TB and the IAC valve. Don't know if its the IAC valve sticking or something in the PCM???? Any thoughts would be greatly appreciated.
Thanks,
Bill
'96 LT4, ZF6, Cam, headers, 58MM TB, starts and runs great.. New development... last couple of times, after running thru the gears 1-2-3 at WOT the motor will not return to normal idle (approx. 900RPM) but hangs at 1200RPM. I can blip the throttle several times (and the TB blades seem to close completely as the linkage on the TB solidly hits the stop screw), with no change in the high idle so I don't believe this is a mechanical issue. If I turn off and restart inmmediately, the Idle returns to normal on restart. I've R&R'd and cleaned the TB and the IAC valve. Don't know if its the IAC valve sticking or something in the PCM???? Any thoughts would be greatly appreciated.
Thanks,
Bill
Last edited by billsc1; Jan 3, 2014 at 12:29 PM.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
you have an air leak. Vac leak.
If the IAC is functional when the a/c is turned on/off at idle,
idle will go up when turned ON and idle goes down when a/c turned OFF....if it does that the IAC is likely fine.
If it is suspect, just pull it, clean the TB passage IAC pintle and TB seat (BUT DO NOT TRY TO ADJUST by TWISTING OR PUSHING ! ! !)
You already did that, so......
back to the 1st line.
trace vac leak. Rule out the TB shafts 1st by spraying wd-40 on the shaft to make it seal for the moment...then look at lines, fittings, egr etc asap ok?
If the IAC is functional when the a/c is turned on/off at idle,
idle will go up when turned ON and idle goes down when a/c turned OFF....if it does that the IAC is likely fine.
If it is suspect, just pull it, clean the TB passage IAC pintle and TB seat (BUT DO NOT TRY TO ADJUST by TWISTING OR PUSHING ! ! !)
You already did that, so......
back to the 1st line.
trace vac leak. Rule out the TB shafts 1st by spraying wd-40 on the shaft to make it seal for the moment...then look at lines, fittings, egr etc asap ok?
Race Director
Joined: Nov 2003
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From: Overland Park Kansas
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IAC is a possible candidate. The easiest way to confirm/deny would be viewing IAC steppers counts.
Racer
Joined: Jun 2010
Posts: 365
Likes: 27
From: Shelby Township Michigan
I had a high idle situation happen to me this past summer after going down the quarter mile. I also thought it was the IAC, but I broke the TPS. If you remove the bolts that hold the TPS and are able to turn it without any resistance it's broken and needs replaced.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
Thanks Lee..I thought about a vac leak but wouldn't it leak all the time and not just after a WOT blast when there is no vac?? Maybe it could be the surge from the restored vac after WOT snaps shut. Brake clean here I come!!
you have an air leak. Vac leak.
If the IAC is functional when the a/c is turned on/off at idle,
idle will go up when turned ON and idle goes down when a/c turned OFF....if it does that the IAC is likely fine.
If it is suspect, just pull it, clean the TB passage IAC pintle and TB seat (BUT DO NOT TRY TO ADJUST by TWISTING OR PUSHING ! ! !)
You already did that, so......
back to the 1st line.
trace vac leak. Rule out the TB shafts 1st by spraying wd-40 on the shaft to make it seal for the moment...then look at lines, fittings, egr etc asap ok?
If the IAC is functional when the a/c is turned on/off at idle,
idle will go up when turned ON and idle goes down when a/c turned OFF....if it does that the IAC is likely fine.
If it is suspect, just pull it, clean the TB passage IAC pintle and TB seat (BUT DO NOT TRY TO ADJUST by TWISTING OR PUSHING ! ! !)
You already did that, so......
back to the 1st line.
trace vac leak. Rule out the TB shafts 1st by spraying wd-40 on the shaft to make it seal for the moment...then look at lines, fittings, egr etc asap ok?
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
All good suggestions so far and things I would do as well.In my experience with all the other cars I own that uses the IAC motor, if I ever suspect it might be a problem I replace it (no messing around).
I have cleaned them and had success for a while but the fix never seems to be permanent for me. Once they start to act up or become intermittent, replacement seems to be the only cure.
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Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
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From: Sandy Springs Georgia
Idle does compensate when AC is energized/de-energized so to that extent the IAC circuit seems to work.
Le Mans Master
Joined: Aug 2007
Posts: 8,735
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From: WV
I was just thinking, (very dangerous), that you said when you turn the key off and restart the car that it runs normal...This would indicate electrical not mechanical..So Im thinking maybe it could be something like a bad TPS or IAC...These would reset when the car was shut off and restarted..A bad ecm could also cause this but I would consider that last...Do you know how to check for a bad TPS???.WW
Last edited by WW7; Jan 3, 2014 at 01:57 PM.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
I was just thinking, (very dangerous), that you said when you turn the key off and restart the car that it runs normal...This would indicate electrical not mechanical..So Im thinking maybe it could be something like a bad TPS or IAC...These would reset when the car was shut off and restarted..A bad ecm could also cause this but I would consider that last...Do you know how to check for a bad TPS???.WW
Race Director
Joined: Apr 2002
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From: Anaheim CA
The PCM has what is known as "throttle follower" mode. When you mash on the gas pedal the PCM opens the IAC valve in sync with the TPS. The purpose of this is to keep the engine from stalling when you let off on the gas pedal and the throttle blades slam shut.
It's supposed to "decay" and let the idle drop back down to normal. It sounds like that isn't happening in your case.
The suggestions to check TPS seem appropriate here. Measure the voltage on the center pin of the connector. It might be easier to just replace it.
It's supposed to "decay" and let the idle drop back down to normal. It sounds like that isn't happening in your case.
The suggestions to check TPS seem appropriate here. Measure the voltage on the center pin of the connector. It might be easier to just replace it.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
The PCM has what is known as "throttle follower" mode. When you mash on the gas pedal the PCM opens the IAC valve in sync with the TPS. The purpose of this is to keep the engine from stalling when you let off on the gas pedal and the throttle blades slam shut.
It's supposed to "decay" and let the idle drop back down to normal. It sounds like that isn't happening in your case.
The suggestions to check TPS seem appropriate here. Measure the voltage on the center pin of the connector. It might be easier to just replace it.
It's supposed to "decay" and let the idle drop back down to normal. It sounds like that isn't happening in your case.
The suggestions to check TPS seem appropriate here. Measure the voltage on the center pin of the connector. It might be easier to just replace it.
Thanks Cliff... After cold soak - Key on/engine off voltage at blue TPS wire with blades closed is .84DCV and at WOT 4.65VDC. I'll blast it around later and see if I can duplicate the condition and then check voltage again. Am I close????
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
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From: Sandy Springs Georgia
Thanks Frankie, got a base line of .84DCV closed and 4.65 WOT key on engine off. Will see what it is when its happening!
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
I would get a comparison from someone else who has a 96 or check the FSM for a 96.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
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From: Sandy Springs Georgia
On my 94 the TPS voltage at idle has always been 0.64 volts. I think my FSM indicates 0.63. I know many other have measured the same however I realize yours is an 96. It may be different on a 96 but if it is not, then your closed voltage might be to high.
I would get a comparison from someone else who has a 96 or check the FSM for a 96.
I would get a comparison from someone else who has a 96 or check the FSM for a 96.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
I'm thinkin vac leak causing the high idle after WOT 'cause...
WOT is a no or low vacuum condition that makes dramatic changes in fuel/timing and everything...then suddenly it goes to idle again according to sensor inputs BUT its still getting some air from somewhere that's unmetered..(vac leak). The radical transition from WOT to idle sometimes causes small leaks to have big effects. The ECM can compensate until the WOT where the little leak suddenly got to be...WOT.
Knowing how fragile these hard-lines & hoses are and the many places there are for a leak to happen, that's my best guess. A TPS issue would be next, and yes, Cliffs explanation of the throttle-down is very good as well. THAT feature used to be handled by a "throttle-dashpot" on carbs to prevent the butterfly from slamming shut after WOT causing a stall especially in cars with a manual trans. A program in the ECM would be the equivalent in these computer controlled engines.
WOT is a no or low vacuum condition that makes dramatic changes in fuel/timing and everything...then suddenly it goes to idle again according to sensor inputs BUT its still getting some air from somewhere that's unmetered..(vac leak). The radical transition from WOT to idle sometimes causes small leaks to have big effects. The ECM can compensate until the WOT where the little leak suddenly got to be...WOT.
Knowing how fragile these hard-lines & hoses are and the many places there are for a leak to happen, that's my best guess. A TPS issue would be next, and yes, Cliffs explanation of the throttle-down is very good as well. THAT feature used to be handled by a "throttle-dashpot" on carbs to prevent the butterfly from slamming shut after WOT causing a stall especially in cars with a manual trans. A program in the ECM would be the equivalent in these computer controlled engines.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
I'm thinkin vac leak causing the high idle after WOT 'cause...
WOT is a no or low vacuum condition that makes dramatic changes in fuel/timing and everything...then suddenly it goes to idle again according to sensor inputs BUT its still getting some air from somewhere that's unmetered..(vac leak). The radical transition from WOT to idle sometimes causes small leaks to have big effects. The ECM can compensate until the WOT where the little leak suddenly got to be...WOT.
Knowing how fragile these hard-lines & hoses are and the many places there are for a leak to happen, that's my best guess. A TPS issue would be next, and yes, Cliffs explanation of the throttle-down is very good as well. THAT feature used to be handled by a "throttle-dashpot" on carbs to prevent the butterfly from slamming shut after WOT causing a stall especially in cars with a manual trans. A program in the ECM would be the equivalent in these computer controlled engines.
WOT is a no or low vacuum condition that makes dramatic changes in fuel/timing and everything...then suddenly it goes to idle again according to sensor inputs BUT its still getting some air from somewhere that's unmetered..(vac leak). The radical transition from WOT to idle sometimes causes small leaks to have big effects. The ECM can compensate until the WOT where the little leak suddenly got to be...WOT.
Knowing how fragile these hard-lines & hoses are and the many places there are for a leak to happen, that's my best guess. A TPS issue would be next, and yes, Cliffs explanation of the throttle-down is very good as well. THAT feature used to be handled by a "throttle-dashpot" on carbs to prevent the butterfly from slamming shut after WOT causing a stall especially in cars with a manual trans. A program in the ECM would be the equivalent in these computer controlled engines.
Thread Starter
Instructor
Joined: Dec 2011
Posts: 191
Likes: 3
From: Sandy Springs Georgia
Fixed... the IACs have it!
Thanks LEE, here's where I'm at. The TPS readings are within specificed range albiet a bit on the high side closed and at WOT. If the IAC goes wide open (fully retracted) in sync with a WOT signal from TPS then it should close (extend) slowly (bleeds down)when it gets the closed signal from TPS. I think the IAC is staying open as bliping the throttle while at high idle (wide vac fluctuation) does not have any effect however stopping and restarting returns to normal idle. This and a can of brake clean and a can of starting fluid failed to show up any vac leaks leads me to believe it's electrical and not vac related. Just ordered a new IAC and will follow up when it's installed.
Bill