IAC reset problems
08-20-2017, 07:33 PM
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IAC reset problems
89TA, 305, TPI, 300k plus original miles, still my daily driver.
Lately I have had high idle issues. Occasionally when I start it up (when hot) It will immediatly race to 2k+ rpm and never goes back down. That was an intermittent problem. Now, every start hot or cold it is steady at 1200rpm.
If I do an eco pin out to a and b, reset and start, its perfect at 600-650 rpm. Turn it off and restart, it'll be back at 1200.
It seems like everytime I reset it, it wants to go back to 1200rpm.
This is a new IAC. I'm puzzled. Vacume leak perhaps? Just seems odd since it will reset to normal manually.
Any ideas?
Lately I have had high idle issues. Occasionally when I start it up (when hot) It will immediatly race to 2k+ rpm and never goes back down. That was an intermittent problem. Now, every start hot or cold it is steady at 1200rpm.
If I do an eco pin out to a and b, reset and start, its perfect at 600-650 rpm. Turn it off and restart, it'll be back at 1200.
It seems like everytime I reset it, it wants to go back to 1200rpm.
This is a new IAC. I'm puzzled. Vacume leak perhaps? Just seems odd since it will reset to normal manually.
Any ideas?
08-25-2017, 04:56 PM
#3
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Car: 87 Monte Carlo SS
Engine: 89 350TPI Transplant
Transmission: 2004r
Axle/Gears: 3.73
Re: IAC reset problems
300,000 original miles. I am impressed. GM should give you some kind of recognition.
Concerning your problem, since the IAC is new and with the mileage you have, I would suggest doing a pin-out from the iac connector to the ecm or wherever those wires go. It is possible that a wire might be damaged. Closely inspect the connector. I have seen old connectors that have the insulation pulled back from the connector and the bare wires shorting out. I think this happens due to heat and age. Good luck.
Concerning your problem, since the IAC is new and with the mileage you have, I would suggest doing a pin-out from the iac connector to the ecm or wherever those wires go. It is possible that a wire might be damaged. Closely inspect the connector. I have seen old connectors that have the insulation pulled back from the connector and the bare wires shorting out. I think this happens due to heat and age. Good luck.
08-26-2017, 09:32 AM
#4
Re: IAC reset problems
If you read the ECM data live, what is the target idle RPM when this occurs?
My guess is the IAC is working, and is only doing what it is being instructed to do.
My guess is the IAC is working, and is only doing what it is being instructed to do.
08-26-2017, 12:00 PM
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Re: IAC reset problems
Vader
I agree. I can't read live data so I can't answer anything there. I'm guessing I have enough vacume leaks to allow it to run normally without the iac....lol.
I do have one question tho. With the key off, I'm guessing the iac goes fully closed.
When the engine is running, does the iac have a software minimum stop? Meaning if it's running, it never goes fully closed?
I agree. I can't read live data so I can't answer anything there. I'm guessing I have enough vacume leaks to allow it to run normally without the iac....lol.
I do have one question tho. With the key off, I'm guessing the iac goes fully closed.
When the engine is running, does the iac have a software minimum stop? Meaning if it's running, it never goes fully closed?
08-27-2017, 09:18 AM
#6
Re: IAC reset problems
With the ignition off the IAC does nothing. Newer model year GM cars operate differently, and the IAC can step open after shut-down, but not this vintage.
The fact that the IAC has been replaced and the system is evidently still not operating as expected might indicate that there was nothing wrong with the original IAC, and that there is nothing wrong with the current IAC.
When the ignition is turned on the ECM opens the IAC a defined (parameter) number of steps. The ECM also samples the CTS and adds steps to the IAC "open" command routine as necessary. There are also IAC step adders for when the transmission is in Park/Neutral, for when the A/C compressor is on, and for when the electric cooling fan is turned on.
It is possible that the IAC can close fully during normal operation if the target idle RPM is lower than the indicated RPM. That is precisely why there is a minimum throttle position specification which should be checked/set as part of a thorough tune-up. The 600-650 RPM idle hot is too high for a minimum air position after closing the IAC and disconnecting it.
Apparently, you have proven that the IAC can fully close by inserting a jumper into the ALDL between terminals A and B and turning on the ignition for a few seconds. This forces the ECM to fully close the IAC. In this condition, a car with auto trans at full temperature should idle at about 400-450 RPM in DRIVE. A manual trans car should idle at 450-550 in neutral. Once the IAC is connected and active the ECM will control RPM at the desired point.
It might help to read through this:
https://www.thirdgen.org/forums/tpi/...227165-6e.html
The fact that the IAC has been replaced and the system is evidently still not operating as expected might indicate that there was nothing wrong with the original IAC, and that there is nothing wrong with the current IAC.
When the ignition is turned on the ECM opens the IAC a defined (parameter) number of steps. The ECM also samples the CTS and adds steps to the IAC "open" command routine as necessary. There are also IAC step adders for when the transmission is in Park/Neutral, for when the A/C compressor is on, and for when the electric cooling fan is turned on.
It is possible that the IAC can close fully during normal operation if the target idle RPM is lower than the indicated RPM. That is precisely why there is a minimum throttle position specification which should be checked/set as part of a thorough tune-up. The 600-650 RPM idle hot is too high for a minimum air position after closing the IAC and disconnecting it.
Apparently, you have proven that the IAC can fully close by inserting a jumper into the ALDL between terminals A and B and turning on the ignition for a few seconds. This forces the ECM to fully close the IAC. In this condition, a car with auto trans at full temperature should idle at about 400-450 RPM in DRIVE. A manual trans car should idle at 450-550 in neutral. Once the IAC is connected and active the ECM will control RPM at the desired point.
It might help to read through this:
https://www.thirdgen.org/forums/tpi/...227165-6e.html
09-12-2017, 10:39 AM
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Re: IAC reset problems
Vader
This may sound a bit whack but 2 things fixed this (and several other) issues. Haven't thought much about why...but it worked.
I was having several recurring/intermittent problems
-Intermittent High Idle, leading me to believe IAC was the culprit
-IAC constantly searching, meaning it was working but air flow was whacked, Vacume leaks
Here's where it gets odd...I was having headlight issues. Meaning I would be driving in the morning and ALL my lights would simply go off....few seconds later they'd come back on. Made for some scary morning drives to work. I was thinking bad headlight relay, random short, bad headlight switch, dimmer switch. Two weeks ago I started out to work and noticed a howling sound that changed in pitch relative to engine RPM. Before I could get 20 miles the car died. No power, wouldn't even turn over. Pulled the alternator, it tested bad across the board.
Replaced the alternator and bingo. Headlight and howling issues vanished.
Out of pure boredom, I checked my TPS...fount it set at .74 mV...adjusted that and my idle issues vanished.
Amazing...even us older guys tend to overlook some of the simplest things.
This may sound a bit whack but 2 things fixed this (and several other) issues. Haven't thought much about why...but it worked.
I was having several recurring/intermittent problems
-Intermittent High Idle, leading me to believe IAC was the culprit
-IAC constantly searching, meaning it was working but air flow was whacked, Vacume leaks
Here's where it gets odd...I was having headlight issues. Meaning I would be driving in the morning and ALL my lights would simply go off....few seconds later they'd come back on. Made for some scary morning drives to work. I was thinking bad headlight relay, random short, bad headlight switch, dimmer switch. Two weeks ago I started out to work and noticed a howling sound that changed in pitch relative to engine RPM. Before I could get 20 miles the car died. No power, wouldn't even turn over. Pulled the alternator, it tested bad across the board.
Replaced the alternator and bingo. Headlight and howling issues vanished.
Out of pure boredom, I checked my TPS...fount it set at .74 mV...adjusted that and my idle issues vanished.
Amazing...even us older guys tend to overlook some of the simplest things.