Thread: Random RPM drop when idle

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  1. #1 Random RPM drop when idle 
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    Anyone know what would cause this, I can drive my 2008 base GP fine for a while... sit idle for a few minutes it will idle around 700ish RPM then suddenly you hear the engine slow down and shake mildly when this happens the rpm drops to about 500 RPM but doesn't cut out.. then it goes right back up like nothing happened... this happens very randomly to me, not sure what causes it to do that... any ideas?
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  2. #2 Re: Random RPM drop when idle 
    GrandPrix Junkie
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    Dirty TB or MAF would be a good start.
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  3. #3 Re: Random RPM drop when idle 
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    Throttle body is completely clean and MAF is cleaned about one a year using MAF cleaner.. this has happened for a few years now and that was always the first thing I did but it never helped... air filter is clean also
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  4. #4 Re: Random RPM drop when idle 
    Turbo is the way to go. BillBoost37's Avatar
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    It could be the AC compressor kicking on. It's common.
    I drink..so consider that when reading my posts.

    2010 Audi A6 Dual IC's
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  5. #5 Re: Random RPM drop when idle 
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    Quote Originally Posted by BillBoost37 View Post
    It could be the AC compressor kicking on. It's common.
    No, I've been down that route before and this also happens when the heat is on and the blower actuator is set to blow of the forward vents... I think the AC clutch only kicks in when you have it set to blow down and/or to the windshield.. at least that's the only time I've heard it come on when it's in heat mode with the AC turned off..
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  6. #6 Re: Random RPM drop when idle 
    GTX Level Member EJOUIE's Avatar
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    My car does this also. The nature of the drop is similar to that of when the car is warming up (if it is indeed the same issue). Seems like the PCM is testing the waters at 500, finds out they are rough, and then retreats back to safety that is 700 rpms. Eventually it will find a good idle rpm (lower than 700 and above 500) but that last bit has some trial and error.

    Same thing or no? If it is then its normal to my knowledge.
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  7. #7 Re: Random RPM drop when idle 
    Turbo is the way to go. BillBoost37's Avatar
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    I'd have to check the manual to confirm if it can cycle w/o being on defrost. However mine does cycle plenty and I don't have it set to defrost, yet the auto will send it there and floor. If I get a chance..i'll check.
    I drink..so consider that when reading my posts.

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  8. #8 Re: Random RPM drop when idle 
    GrandPrix Junkie
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    My experience w/ these things says it's normal as well. The only W-body I haven't seen do this was my 5 speed swap...A) the A/C didn't work, and B) I tuned it out.
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  9. #9 Re: Random RPM drop when idle 
    Killa Bee Scottydoggs's Avatar
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    mine does this too. tap the gas and the volts go up. gm alts seem to put out less volts at idle, vs when driving.

    so you come to a stop light and the fan power dips a bit then comes back up. turn the steering wheel at a dead stop same thing will happen. volts drop a bit.

    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, gen 5 N* 3.0 pulley, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW
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  10. #10 Re: Random RPM drop when idle 
    Turbo is the way to go. BillBoost37's Avatar
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    Scotty needs to look under why that happens in accessories under DHP.
    I drink..so consider that when reading my posts.

    2010 Audi A6 Dual IC's
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  11. #11 Re: Random RPM drop when idle 
    Killa Bee Scottydoggs's Avatar
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    do tell. all ears.

    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, gen 5 N* 3.0 pulley, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW
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  12. #12 Re: Random RPM drop when idle 
    Turbo is the way to go. BillBoost37's Avatar
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    Quote Originally Posted by BillBoost37 View Post
    Scotty needs to look under why that happens in accessories under DHP.
    Cough cough
    I drink..so consider that when reading my posts.

    2010 Audi A6 Dual IC's
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  13. #13 Re: Random RPM drop when idle 
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    What I have come across with 3.1, 3.4, 3.8, now my 3.9 g6 is vacuum leak on lower intake. Try spraying carb cleaner around lower intake when cooled and see if you get rpm rise when spraying it. Good luck..
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