Idle speed erratic 98 TA
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Staging Lane
Joined: Oct 2011
Posts: 75
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Idle speed erratic 98 TA New problem with the idle speed.. Yesterday my battery was dead due to me not shutting off the dash cam over the last 4 days. Jump started it with a battery charger. I noticed that the idle speed was very high about 1800 RPM and it took at least 1 minute to drop at all. In the meantime the idle was also fluctuating up and down. I had a code P0507 indicating a probable IAC valve performance failure. So I replaced the IAC valve with a new one and the code has not reappeared but not much has changed. When I do a cold start, the idle speed now goes to about 1500 and sticks there for about 10 seconds before dropping to normal ~900. This is new. Before, the idle speed cold was about 1200 and it took only a few seconds to drop. I performed the idle speed reset described in the FSM. No change. After the engine is warmed up for at least 10 minutes, it all settles down but before that, if I switch on AC or shift in or out of gear, the idle speed jumps up temporarily much more than before. I am thinking maybe a vacuum leak somewhere (PCV, EVAP purge valve...?). Perhaps someone had this specific issue and can describe their experiences before I start checking parts. Thanks.
98 Trans Am LS1, stock
98 Trans Am LS1, stock
TECH Resident
Joined: Jul 2007
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From: Winter Haven FL
After disconnecting the battery you have to do the idle relearn. It takes about 15 minutes and is pretty easy. It'll basically tell the pcm where the idle should be for all the different scenarios.
Joined: Nov 2001
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From: Schiller Park, ILL Member: #317
P0507 doesn't necessarily mean that the IAC is bad, it just indicates that idle speed is higher than commanded (I think the threshold is ~200rpm above commanded idle speed, but I can't remember the duration threshold) - and that could be happening for a few different reasons.
So, if everything was fine before the battery ran down and P0507 was the only code, then I wouldn't start by changing the IAC. This adds a new variable and it's possible that the new IAC is faulty or of lesser quality. Did you replace the IAC with a new GM piece? I'm not sure who GM is currently using to supply these, so it may or may not be better than other brands or the original that you replaced.
Having said that, I would start over with the previous IAC motor, since it was fine before the battery ran down. I would then have the battery tested (since it was dead and you didn't mention how old it is; frankly, if it was mine I would just replace it unless it was pretty new) to make sure you're not fighting with odd issues related to an old battery that may have suffered damage due to being run completely down. Then, with a fresh battery I would do a PCM reset, clear any pending/stored codes, and then do a proper idle relearn as described in the service manual.
At that point, if there are still issues then it would make sense to start looking for vacuum leaks or suspect some type of failure of a sensor or valve. But it would be highly coincidental for this issue to be related to anything other than the dead battery with which the whole issue began.
So, if everything was fine before the battery ran down and P0507 was the only code, then I wouldn't start by changing the IAC. This adds a new variable and it's possible that the new IAC is faulty or of lesser quality. Did you replace the IAC with a new GM piece? I'm not sure who GM is currently using to supply these, so it may or may not be better than other brands or the original that you replaced.
Having said that, I would start over with the previous IAC motor, since it was fine before the battery ran down. I would then have the battery tested (since it was dead and you didn't mention how old it is; frankly, if it was mine I would just replace it unless it was pretty new) to make sure you're not fighting with odd issues related to an old battery that may have suffered damage due to being run completely down. Then, with a fresh battery I would do a PCM reset, clear any pending/stored codes, and then do a proper idle relearn as described in the service manual.
At that point, if there are still issues then it would make sense to start looking for vacuum leaks or suspect some type of failure of a sensor or valve. But it would be highly coincidental for this issue to be related to anything other than the dead battery with which the whole issue began.
Thread Starter
Staging Lane
Joined: Oct 2011
Posts: 75
Likes: 7
I have already done everything you mentioned except replace the battery. It was not fully discharged, just not enough charge to start the car. It is only 1 year old and after charging, it is fine. Reinstalled the original IAC valve, reset the pcm. No stored, live, or pending codes according to my Tech II, And did the idle relearn.
I am going to search for a scan of the engine before this happend and compare the IAC position at normal idle speed with what I see now. If different, I will check the PCV and EVAP purge for leaks. Last resort will be to swap the pcm with my spare.
I am going to search for a scan of the engine before this happend and compare the IAC position at normal idle speed with what I see now. If different, I will check the PCV and EVAP purge for leaks. Last resort will be to swap the pcm with my spare.
Joined: Nov 2001
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From: Schiller Park, ILL Member: #317
I still wonder how the battery would perform under a load test at this point, even at just one year old it may not have been the best battery in the first place (marginally faulty, old off the shelf and/or poor storage conditions prior to sale, etc.). Some of the learned idle parameters may be getting lost or corrupted during a cold start if the battery is in a weakened state (even if there is still enough juice to start the engine). This could explain why it takes about ~10 minutes or so for everything to settle down after each cold start (time for any lost/corrupted idle parameters to be relearned), but that it does eventually settle down.
Do you have similar problems after a warm/hot restart? Or just from cold?
I don't think you'll find the answer in the PCV or EVAP system. I had a high idle/idle hang issue relating to the EVAP purge valve on one of mine, it only caused a high idle when warm (and only after extended cruise periods, normally at expressway speeds) and it would never correct itself, no matter how long I let it run, without cycling the ignition. I'm sure there's more than one failure mode for the EVAP purge but, again, it would be a mighty big coincidence that it happened to fail at the same time as the battery discharge. I'm thinking this will be related to weakening of the battery hastened by damage during the discharge, or perhaps an issue tied to something that went awry during the jump start process.
Do you have similar problems after a warm/hot restart? Or just from cold?
I don't think you'll find the answer in the PCV or EVAP system. I had a high idle/idle hang issue relating to the EVAP purge valve on one of mine, it only caused a high idle when warm (and only after extended cruise periods, normally at expressway speeds) and it would never correct itself, no matter how long I let it run, without cycling the ignition. I'm sure there's more than one failure mode for the EVAP purge but, again, it would be a mighty big coincidence that it happened to fail at the same time as the battery discharge. I'm thinking this will be related to weakening of the battery hastened by damage during the discharge, or perhaps an issue tied to something that went awry during the jump start process.
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Thread Starter
Staging Lane
Joined: Oct 2011
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Anyone know what their IAC position is at idle? Mine is about 35 and that seems low if I am remembering correctly.
10 counts at 575 RPM, 100 counts at 1500 RPM.
10 counts at 575 RPM, 100 counts at 1500 RPM.
Last edited by GaryDoug; Apr 24, 2019 at 05:22 PM.
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Verdad Gallardo Joined: Nov 2001
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From: Schiller Park, ILL Member: #317
Factory commanded idle speed (hot) for all LS1 F-body A4 cars is 550rpm in gear, 650rpm in P/N; for M6 cars it's 800rpm.
IAC should be at 0 if the idle speed is higher than commanded (this would illustrate the PCM's attempt to bring idle back under control if too much air was entering via some other source) and the engine is actually at idle (meaning TPS @0.0-0.4%, ~0.5v). If idle speed is elevated and the IAC is not at 0, then the PCM is not recognizing this discrepancy or something is happening to make it believe that the engine is not actually at idle (such as elevated TPS values).
As asked above, do you experience similar problems after a warm/hot restart, or are there only issues immediately after a cold start?
Thread Starter
Staging Lane
Joined: Oct 2011
Posts: 75
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Running perfectly today, no idea why. Yesterday I swapped the pcm and had no improvement. Left the spare pcm connected overnight. This morning I swapped back to the original pcm. As of now, everything is original. When I started the engine, it ran perfectly. I did not perform the idle speed relearn this time because it worked fine in or out of gear and with the ac on or off. Besides, I didn't want to jinx anything. Drove it around for half hour. Let it sit for 4 hours and it started and ran perfect. Maybe there is something intermittent like a connector, valve or hose.