Rough Idle w/ AC (ocassionally stalls)

My car is a 2004 Chevy Trailblazer. 6 inline, 4.2L

Been having an issue with my the idle for the last year.

Quick description: Car runs perfect as far as I can tell when moving, or when idling with the AC off. When the car is idling and the AC is on, the rpm fluctuates around, and occassionally will dip down low like it’s about to stall (and in a few cases does), but then it jumps back up. The handful of times the car does stall, it starts up again right away without issue. At no point have I ever gotten a single code when this has happened.

Repairs that have been made to my car in the last year. Not all of these are from my attempts to fix the rough idle, but I’ll just list them all.

Oil & filter and airfilter have been changed on time.
Removed and cleaned throttle body (removed it again later to check make sure)
Replaced gasket between throttle body and intake manifold
Replace MAP and Air Intake Temperature sensor.
Replaced engine mounts
Replaced AC relay
Replaced spark plugs
upgraded PCM firmware
Replace variable timing solenoid
Replaced battery
Replaced fuel filter

At this point I really don’t know what else to try. I’ve spent days searching for a solution, but googling “rough idle” just gives me 10,000 results saying “clean your throttle body”.

Just to summarize. Car only runs rough when it’s idling, and the AC is on.

Would really appreciate any help anyone can provide.

Your engine uses an electronic throttle body…

And when cleaning an electronic throttle body, the procedure is different.

Watch this.


Well he’s correct, I didn’t clean it the proper way though.

I’ve done over a dozen pcm resets over the last year. What usually ends up happening is that it works well for a few days, maybe a week, then the idle starts to get rough again.

Again, just to clarify, without the AC on, it’s rock steady at 600rpm. It’s only when I turn on the AC that it starts acting up. I’m not arrogant enough to say it’s not the throttle body, I’m just very doubtful at this point.

I’ll take another crack it. If it doesn’t work, I’ll search for alternate relearning procedures.

Was really hoping I would have this solved but sadly still no luck.

So getting fed up with the possibility it was the throttle body, I finally said f*@k it and bought a brand new one several months ago. I was careful not to push on the throttle body plate at all. Also went ahead and replaced the gasket again as well. Still no dice.

I started noticing that while the idle was really only rough when the AC was on, it also seemed to happen primarily after the engine had warmed up. Started doing some digging and turned up 2 issues I could do myself immediately (yes, I know there are a lot of things in the engine that heat affect).

  • Camshaft position sensor
  • Oxygen sensors

Replaced the cps, and while I was working on replacing both of O2 sensors, I noticed that there were a few small holes in the tailpipe a few inches below the downstream O2 sensors. Put some of the high heat quiksteel putty on them.

And again, still the same damn problem.
At this point I am at least confident the trailblazer isn’t going to just quit on me and never start back up again, but damn I would love to get this solved.

One thing I did notice is that the drive belt tensionser wobbles a small but noticeable amount. Can’t find anywhere online indicating how much is too much, so I think I might replace that next just in case.
My biggest fear is that it’ll be the ac compressor, but the ac always worked fine for me.

Anyone got any other ideas?

I have the same issue, were you able to find out what the problem was? Thanks