Rough Idle P300
 

I have a 2011 Chevy HHR when I start it up and turn on the heat all the way I have no issues, I can let the car idle for over a hour, when I turn off the heat in about 20 minutes the idle gets slightly rough but no stalling the car runs around 160 to 170 in this condition. If I start and run the car with no heat within about 20 minutes I will get a very rough idle and the car will stall, the temperature range is usually around 180 when this happens and it has the dreaded P300 code any help is appreciated

:welcomewagon: to the forums!

Yep, if you're only getting 160-170 degrees, you have a failed thermostat. Use only ACDelco part number 131-158

And change the Dexcool while you’re at it, a simple drain and refill with 50/50 Dexcool and potable water, you can buy it premixed, and burp the coolant system to get all the air out

https://www.chevyhhr.net/forums/how-...l-p0128-24493/

https://www.chevyhhr.net/forums/how-...-system-60614/

In addition to cleaning the throttle body and MAF sensor, I would throw a bottle of fuel injector cleaner in the tank.

It’s a 2 month old thread. One time poster never came back.

And we offered such good advise!

Info received
 

This wasn't my post but I got a lot of good information out of it. Thank you.

I put in seafoam it ran much better and stopped stalling but still a little rough thanks

Then you still need to drain and fill the Dexcool and change the thermostat as suggested in post #2. I also recommend you clean the throttle body and MAF sensor as suggested in post #4

^^This^^

(Maybe a 2nd opinion will help convince.)