2003 GMC Envoy rpm needle goes up and down in idle and revs

gmc

#1

So yesterday I believe is when my GMC Envoy 2003 started acting weird and I didn’t worry at first but now whenever i’m idling even after the car is warmed up the rpm needle with go up and down from about halfway to 1 to almost 0 and goes back up. It does this about every 6 seconds when I’m sitting in the driveway or at a light or stopping and I am worried it will stall. Maybe I need to change the oil? I just filled it up with gas and thought it might help somewhat but it still does it. My car has about 212k miles on it and the check engine light has been on for a little while, it was on before and we had to replace the MAP sensor according the the code reader and that is what we did. It was fine awhile after that but now this problem is happening and everything the rpm needle goes down and back up the car shakes and the engine revs itself but it doesn’t do it all while I’m driving. I have been keeping the heat on everyday because of the cold and I don’t know if heat or AC can cause this problem. Please let me know if you might have some advice for me or what my car might be doing, also i don’t really know much about cars and my best guess would be a vacuum leak or some other kind of leak but thank you.


#2

What code is causing the check-engine light now?


#3

There are many potential causes. The best first step is to get the codes read again as this will likely point in the right direction for what to check. I’ve owned three Trailblazers (the sister to the Envoy) an '03, '05 and now an '08 so quite familiar with the truck. Post back with the codes and we can go from there.


#4

okay thank you, once my dad gets home i’ll have him check my codes and i’ll let you know.


#5

Mine was going stupid rpm surge even while driving, only with the ac or defrost on. bud said reset the brains, disconnect battery for 30 seconds, and all was good. Make sure you have all climate control systems off, as that may lead to actuator failure for hvac. No codes for me.


#6

update: This morning driving to work I noticed that when I turned the heat off in my car it would stop acting weird, it’s still seemed to drive a tad bit rough but i think it’s always been like that I just usually don’t notice it maybe. But now I know it has something to do with A/C, I’ll check my fluids when i get back home after work just to see.


#7

another update, i’ve just figured out that my car being on defrost was making it do that so i can still use my heat without defrost and the car will act normal, I still don’t know the root of the problem if anyone knows anything?


#8

thank you i’ll try it


#9

Your engine control is unable to react to loads. When defrost is on, the A/C compressor will run periodically to dry the air. That is a big load on the engine. Normally, the computer will bump up the idle to compensate but for some reason yours is not reacting properly. On some cars, this is the function of an IAC but the TrailVoy platform does this with the throttle body and these are known to be needing periodic cleaning. This would be the first thing I would check. Cleaning the TB is relatively straightforward on these…