Just as a correction. Walnut cleaning is done on intake side not exhaust.
How many miles and what was the code?

 

Stop throwing parts at it and please give us the codes.
And just for grins and giggles, why did you replace the coils and injectors?
Did you check the wire harness connections to the injectors, coils?
Did you replace the spark plugs?
Have you checked for a stuffed up air filter?
Run sea foam thru the fuel tank?
Is it possible you got a bad tank of fuel?
Could your fuse box be loose?


https://www.chevyhhr.net/forums/2-0l...-thread-54368/

 

You started another thread about a misfire but you never came back to follow up on that.
Is this the same issue ? If so, the other thread has some added info regarding a tune.
Other thread=https://www.chevyhhr.net/forums/prob...misfire-59467/
And another thread. Same type problem. OP never came back.
https://www.chevyhhr.net/forums/hhr-...ng-tune-58692/

 

Merged
If your intentions were to either update or maybe y’all couldn’t find your other threads here they all are, now answer some questions that have bee asked in all the threads

 

The motor has low miles since it was replaced but all the other parts had 178K miles. So i have done a mini restore. Changed the Injectors, Coil Packs, High-Pressure pump, Manifold and Throttle Body. No leaks as i can find in the Turbo charge pipe. Building Pressure is no problem but at 10 PSI it starts to misfire and get worse as the pressure builds . Removed the Tune back to stock also, no change. Low-pressure pump is at 45-50 PSI. Changed plugs multiple times

 

What’s the compression numbers?

 

How about a dump valve (or whatever it's called) malfunction?

 

Don. It is technically a bypass valve. Could be dumping boost. Not much to break in there and it's easy to pull and check.

 

He said he put on an aftermarket blow off valve. But that might mean a Forge bypass valve.

If the engine is very low miles, and the valves are caked, you might have a problem with excessive crankcase pressure. Maybe rings/cylinder walls or damaged piston/ringland. Like oldblue said, you need a compression test and possibly a followup leak down test. How did you see that the valves are caked?

And please confirm the code(s), is it P0304? Any others?

There are several possible causes, we will have to work through them.

 

The motor has 35K miles since the original developed a crack and the dealer replaced it under warranty. But all the other components had 180K miles. I drive serious miles every day. So I made the decision to not but a new car and instead do a mini overhaul. Also bought all the parts for a serious bargain. The misfire problem has stayed even with all the new parts. I could see the valves when i changed the manifold. So it received Injectors, Coils, Manifold, Throttlebody, High pressure pump, Catch Can, Alternator, All belts and Hoses, and Plugs. As well as complete front end parts. I had purchased a non-cat pipe over a year ago but every time i try to use it i get lots of white smoke when i come to a traffic light, the smoke builds and builds with no obvious resion. when moving no smoke. When i reinstalled the original pipe no smok. Only thing i can come up with for that issue is something with the turbo. Anyway the main problem is the misfire at 10 PSI and above. Im going to change out the NGKs to Delco again to see what happens. And a new dump valve. Compression check next.