Im working on my sons 2011 HHR with 2.2 and its driving me nuts. It will intermittently run rough but not all the time. Only code is p0300 misfire detected. I thought I had found the problem when I found the timing chain was loose but after replacing it there is no change. Today I checked fuel pressure and with engine running it fluctuates rapidly between 50-55 psi? Is this normal?

 

Are you sure it's timed correctly?

 

I don’t see why the fuel pressure should be bouncing around. How are you measuring it? What is the fuel pressure do when it’s not running? (The fuel pump only runs for a couple seconds if you turn on the ignition without starting it. There is an internal check valve that should hold around 58 psi for at least several minutes.)

So please verify, you seem to be saying the only code is C0300, and that hasn’t changed since the timing chain job?

How many miles? Any other current issues or significant service history?

 

He said p0300. Not C0300.

 

Put a vacuum gauge on it and if it's bouncing like the FP then I suspect it's out of time.

 

Out of time would trip a code, most likely P0016

 

Im pretty confident It is timed correctly. I triple checked all timing marks before I buttoned it up. Im measuring with an OTC fuel gauge, pressure is steady with key on but engine off but then fluctuates rapidly with engine on. After key is off pressure stays steady for at least 15 minutes.
The P0300 code was present before TC, I erased code but it came back after TC. Car runs exactly the same as before too. It starts great but after warm up it will intermittently run really rough and then run fine. Thats the part that has me puzzled.

 

Do a compression test & leak down if indicated. Look at the entire length of the fuel lines, for kinks or holes. Check the EVAP purge valve, it is normally CLOSED. Check all of the electrical connections. After that you are looking at a new fuel pump.
My suspicion is the purge valve is intermittently malfunctioning. It is a PWM thing not open/close. The fuel pressure is suspicious but not out of spec.

 

Typo fixed. Not that it made much difference.

 

So I want to thank everyone for their help and advice but I especially want to thank Donbrew because he was absolutely right it was the EVAP purge valve. I would have never come up with that being the problem. changed it out and my sons HHR runs great now so THANKS again donbrew!!!