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Random RPM drop when idle

B

bluebsh

New member
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?
 


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
 
BillBoost37 said:
It could be the AC compressor kicking on. It's common.
Click to expand...

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..
 
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.
 


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.
 
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.
 
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.
 


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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