So this has been an on going issue on this car. 2011 chevy hhr base ls 2.2l.
To date, I've replaced spark plugs, coils, fuel injectors, fuel pump, gas cap, up and down steam o2 sensors, catalytic, intake manifold, purge valve purge valve line (had a leak) and egr pressure valve. All this is less than a year old.

I'm getting code p0172 and p0451. Compression is 118 121 124 121 right to left.
At speed, it runs great. At idle, you can feel it pulsing almost like a misfire. Short from taking it in and having it tested again, what ideas would you guys suggest I check?

 

Clean the MAF sensor with Sensor Safe cleaner, clean the throttle body with throttle body cleaner , not carb cleaner or brake cleaner.
DI not attempt to move the butterfly plate in the throttle body , the plastic gears strip very easy.
Look for a cracked exhaust manifold or a broken flex pipe

 

The P0451 is the problem. The fuel tank pressure sensor is not operating correctly.
It is probably holding the EVAP purge valve open causing the P0172, and I bet hard starting.

 

Those compression numbers seem a little low.
How many miles on this car ?