Low Compression in ALL cylinders and P0300/0303 code
 

Greetings All! Long-time Ford owner, first-time Chevy owner: 2008 Charcoal Gray HHR LS, bought used with 110K, just about to turn 130K.

CEL turned on, scan showed P0303. I changed the plugs (maybe original plugs?) and the misfire remained. I swapped coils #2 & #3; scan showed P0300/0303. I swapped fuel injectors #3 & #4; scan showed P0303.

Drivability does not seem to have changed, but the idle has roughened considerably.

Went to a mechanic for a compression test yesterday. #1 was 140 psi, #2 was 125, #3 was 70, #4 was 140. The mechanic wondered about a stretched timing chain and promised to do research after putting it all back together. He called a contact with GM Fleet and was told the chain was a possibility, but the valves were more likely the issue. He wants to do a leak down test next.

Are we headed in the right direction, or does the low compression across all cylinders with a specific misfire in one cylinder lead anyone to a different conclusion?

You are going in the correct direction. With the low compression on just number 3, I would say the head is coming off, next for inspection. A leakdown can give you a clue, but either way, the head is coming off.

Put "p0303" in the search box, you'll find lots of company.

The #2 #3 thing reads head gasket likely, the book says.

When you have the the head off might as well replace the chain, sprockets, guides and oiler, (comes as a kit) they will be off anyway. Think about the water pump at the same time (sometimes comes in the chain kit).

Low compression usually indicates worn piston rings.

Boydie

But 2 next door to each other and one worse than the other.......The Book says start assUming head gasket, then further test.

Yeah I'm with Don here, adjacent cylinders almost scream bad head gasket, and the service manual backs it up.

Or leaking valve seats, or Head gasket..

I'd suggest you do the 'leak-down' test before any disassembly.. That will at least point ya to top or bottom or even head gasket..

Leak down test results: #1 & #4 good, #2 green @ 30%, and #3 red @ 90%. The air movement was heard through the exhaust pipe. Mechanic is thinking exhaust valves.

I've had this issue on one of my other cars, turned out to be a cracked head near the exhaust valve, but I think we are starting to dig up any scenario, time to pull the head and do a visual check, have the head inspected, and Magnafluxed if possible, while you're at it change out all the timing chain water pump, tensioners gaskets and such, check the piston movement from the top, if they're sloppy, make a decision then to change the motor or pull it for a full rebuild.