I have a 2011 HHR with a 2.2 and 150,000 miles. Bought used last year from a couple girls, who had left it with several problems. I got a great deal!
A couple months ago it started running very ruff, went straight to the plugs and they were very bad. Changed plugs and did install NKG Iridium plugs. Still had a small miss, and very once in a while would throw a P302 code.
I had a couple of ignition coils from my previous HHR. So I changed #2, and it quit throwing a code. While checking everything out, saw the throttle body was black. Rather than clean, I replaced with one from another HHR I owned, but sold after pulling a lot of new parts. Now to my problem. This week in the 100 weather, it started to idle ruff, with the AC on. Turned off AC and it smoothed out.
And then at a traffic light it died, started right back up and drove home. Pulled a code, even though no check engine light was on and it pulled a P0300. Cleared the code and it ran great. Drove it to work and no problem. On the way home, started to idle ruff and died again, throwing the same code again. I feel I know a lot about HHRs since I've had at least one since 2007. But I have never had this code and a situation with it dying while idling. Looking for assitance with Ideas.
Thank for your time!

 

You should have learned that NGK plugs are a mistake. Use only AC/Delco 41-103.
Maybe the AC compressor is getting ready to die.

 

Somewhere here in the forums, someone figured out a belt that could be installed to bypass the AC compressor. You could install it, as a test, and see if the idle improves.

 

^x2^

What’s the engine temp doing though all this? Have you ever tried setting the DIC to coolant temp and monitoring though you drive? (Probably not related, but coolant change may be overdue, btw.)

What p/n NGK plugs? (I understand NGK makes the AC Delco iridiums, but that doesn’t make them the same. Maybe they are, but the AC Delcos are usually cheaper anyway.)

You could try some fuel injector cleaner. Try a tank of premium fuel. Shell or anyone else with “Top Tier.” (Haven’t seen that sticker on a pump in years around here.) Maybe there’s some carbon from a lot of short trips. You could try a long drive to burn it off. Even better with E85 fuel, if they sell it in your area.

I would run a compression check. Even if its fine, write down the results and you’ll have a baseline if you have issues later.

What motor and year did that throttle body come from? Are you sure it’s the right one? I have no idea if they’re all the same. You could try cleaning the original and reinstalling.

 

Clean the throttle body with CRC throttle body cleaner , and the MAF sensor with CRC sensor safe MAF sensor cleaner.
the rough idle and misfires and the mileage tells me to change the timing chain , and the serpentine belt and tensioner ( both, not just the belt )
install the correct AC Delco 41-103 iridium spark plugs.
the 2.2 is a stout engine, be finicky.
check the air filter too!