Rough idle when AC is switched on
 

I've got an 09 SS, based on info from the previous owner it had the GM tune at one point and I believe it has a custom tune at this time since they forwarded me a dyno showing 305whp. I bought the car last october with 76k miles on it and I'm now sitting around 90k. Ever since I bought it, when I turn on the AC the idle gets noticeably rough to the point where you can feel the whole car shake at idle. Switching off the AC or bumping the rpms up to about 1000 with the gas pedal resolves the issue. I've been told this is normal for the hhr by the dealer when I've asked about it, but I doubt this.

Has anyone had a similar issue? I'm not sure if this is just how the car acts, or maybe due to the tune being optimized for idle with AC off, or if my AC compressor is on the way out.

Mine doesn't shake a lot. Does yours have the factory mounts or have they been replaced?

The "tune" has 2 different target idle speed. One with the Air on the other with the Air off. In mine the RPM is the same for both.

The OEM programming in the ECM will increase the engine rpm by 50-150 rpm with compressor engagement, the tune may have modified or eliminated that line of code, leading to the stumble when the compressor clutch engages.

In bone stock form, pressing the A/C button sends a request from the BCM(Body Control Module) to the ECM requesting a compressor engagement, plus a signal to start the cooling fans and adjust the idle speed up a smidge depending on engine temperature...and whether the vehicle is running in Open Loop or Closed Loop mode.

Simple eh? And folks wonder why I dream of setting the points on my 65 Chevy with a dime and a paper matchbook cover.;)

These are my 2008 HHR SS stock idle tables.

https://www.chevyhhr.net/gallery/fil...dle_speeds.jpg

I can tell you that on every other car/truck I have programmed they are indeed different. Again the program for a LNF is a while lot different than anything else.

According to the previous owner the only work done to the car was the GM tune, cat back exhaust and then the custom tune. I believe the mounts are factory but don't quote me on that. The previous owner used to be a member on here, I think he posted under the Dr Jekyll screen name.

It must be the tune then because my idle doesn't go up at all when the AC is turned on. According to the scan gauge I'm sitting at about 780-800 rpm with the AC off and 790-810 with it on. That seems pretty sloppy on the part of whoever did the tune.

On the plus side I have a few extra ponies in my rattle can.

One more question my scan gauge has been indicating -1.5 to -1.00 psi after I shut the car off these days. It used to indicate near zero as it should. Do the manifold pressure sensors go bad over time or need occasional cleaning? I'm not throwing any codes, it just has me concerned that something is up.

Intake manifold might benefit from a cleaning of any crank case oil or moisture sucked in over it's life

I spent a few minutes searching Members (3rd tab from the left in the nav bar) and found:https://www.chevyhhr.net/forums/members/dr_jekyll-8306/

From there you can find all of his posts.

I have a stock 08 SS and it's vibrated with the AC on since I bought it with 70 miles on it. I just thought that's how they were. It has 146,000 on it now and still does it although it might be sightly worse then when it was new. The other thing it does when the AC is on, is that it seems to stumble from a dead stop. I usually have to give it extra throttle to keep it from nosing over then it tends to lunge once it goes. I have no CEL on. The only CEL I've ever had with the car was for the intake cam solenoid which I replaced and hasn't came back on since. Sorry, don't mean to hijack your thread but seems like similar issue...

Thanks,
Keith

Old thread, but I wanted to follow up on this post since I found the issue. After posting this last fall I ended up removing an aftermarket grille the last owner had put on the car. While doing so I found the lower center grille covering the intercooler had a nice hole in it and the intercooler had an indentation which was perfectly wrench shaped. The car made boost at the time so I replaced the grille and left it alone. Fast forward another 8-10 months and my car wouldn't hold boost any longer. Ended up doing a pressure test and the wrench shaped impression was now leaking air like a sieve. I replaced the intercooler with a ZZP unit and my idle with and without the AC on is now smooth. It appears as though the intercooler has had a hole ever since I bought the car which is what caused the rough idle since unmetered air was getting in. Anyways, if anyone has a similar issue with their SS do a boost test.