Backfire!!? I hope you mean something else.
The only way that can happen is valve timing, right? And, if the engine actually turns backwards the chain will jump and valves will bend.
Have you done a compression and leakdown

 

my bad.. misfire. Same misfire code. I do hear a loud pop though
Compression yes.. results are somewhere here, pretty even all 4 cylinders.
Leakdown nope, I have not

 

Have you looked at the timing chain for proper alignment? Maybe a visual on #4 cams. Any unexplained coolant loss/gas in coolant (that would be reason for leakdown) a known issue is a head gasket leak/block porosity between 3 & 4.

 

No, I have not looked at the timing.
could that really get screwed just like that?


As far as coolant, no lose as far as I can tell

 

Anyway... I am getting ready to work on the car in the next 2 or 3 weekends. I also have to replace a fender, so that's more work added to it.


Work:
Replace IC piping, upper and lower because of a leak
Replace injectors seals
Pop valve cover and check


Parts:
Performance Autowerks IC piping
Injector seals kit
Valve cover gasket
Intake gasket
Throttle body/intake seal


Anything I may be missing?
Since I'm at it, I'll look at the valves and see if they need walnut blasting.
I'll move injector 4 to cylinder 3 to see, once everything is back together, IF the injector itself is the issue. Too late?
any tips and tricks? sealants to use with the gaskets?


If anybody is interested, I have and INJEN upper IC pipe for Cobalt, black, new. I wanted to use it, but wasn't aware in the air filter clearance.

 

Did you make the seal tools?

 

YEP!!!


Done, all thanks to you!

 

I am not aware of any gaskets on an HHR that need sealant; well, maybe the oil pan, but in that case the sealant is the gasket.

 

My guess is you may find an injector seal that is bad

 

thanks