Hey guys. Long long time no post. Every time I look something up, it's already been said. I was one of the first few people on here with my 2006 HHR, but only come around now when I need info. This problem is driving me crazy and I am compelled to post.

I've got 270,000 miles on that HHR, but she not the one giving problems. I was recently looking for a used car, and I came across another HHR. Same year, trim, LT, 2.4, only thing different is no sun roof. So, I bought it. I got a better price because I pointed out all the things that needed to be replaced, (control arm bushings trashed of course and definitely sway bar linkage just to name a few). It has 188,000 miles, but I have no idea how someone treated her. Apparently, not well. (Replaced a wheel bearing without the ABS sensor. How much did that save them, 20 bucks? Lol)

I got a P0300. Random misfire. I thought, easy peasy. Well, I can't figure it out. New plugs first. Then new coils. Tested compression. Looked for any vacuum leak. Still P0300. After all the front end work, I needed to get it aligned. I asked the shop, (Firestone) to diagnose the misfire. They said mainly pistons 1 and 3 misfiring. They concluded an exhaust leak before the O2 sensor. I looked for this exhaust leak and really can't find anything. I figured, well let's replace the fuel injectors. Long story, I happened to have a set. They were filthy. Took quite some effort to clean those port holes and pop in new injectors. Still no joy. P0300.

I don't have expensive fancy equipment to look deeper. I use Blue Drive. It has live data. Not sure what I'm looking for. I recorded some data, have some screen shots. It seems ok, but not sure.

It only seems to misfire in Idle. It drives fine. Not sure what to do next. I'm going to take that heat shield off and inspect that manifold where the first oxygen sensor is. Other than that I'm stumped. Any insight? For what sanity I have left?

BTW, got to thank you guys for all the years of information I've gleaned when something comes up. I have appreciated it very much.

 

P0300 does not mean "random misfire" it does mean "misfire detected". I would do compression test first, my guess would be fouled or burned valves.
The Firestone guys either guessed or lied. No way they could tell.
If there was a problem with an O2 sensor there would be other codes.

 

I know right!? I do know with fancy scan equipment you can look at specific cylinder misfires, but yeah I was thinking that too. That doesn't make sense. I'm not getting any O2 sensor codes. Just P0300. I did do a compression test. I wrote the numbers down in sharpie on the valve cover. I'd have to pull the air cleaner and look. I'll post the numbers I got tomorrow. it's dark, I'm tired, lol

 

I can totally believe fouled valves. You should have seen those injectors. Valves... hmmm that's a lot of work. If I got to pull that head I might as well do all that other stuff. .... that's a lot of work. Oh boy.

 

code P0300 may mean that one or more of the following has happened:
Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Faulty camshaft position sensor

The faulty camshaft sensor usually indicates a stretched timing chain.
I’ve seen a dirty fuel rail cause a misfire , did you clean it when you changed injectors?
A cracked exhaust manifold could be the problem, but you would hear the sound coming from under the hood.
What spark plugs did you install?

 

The torque spec for the exhaust manifold bolts is very low. I found mine loose twice. They might even be loose now.

 

No other codes, that rules out cam sensors, cat, O2 Sensor,. Already replaced plugs, coils and injectors.

 

Just 11 ft/lbs for the stud and 10 ft/lbs for the nuts

 

Check to see if the correct spark plugs are used ( ACDelco Iradiums ) put a can of seafoam in the tank and drive it on the highway for a few hours ( add some to the oil as well and change it ) clean out the block of sludge and carbon build up.... but those spark plugs can be the culprit !
Also do a lobotomy and see if its just a stored code, start with a fresh slate.

 

That's what I'm saying, that's only finger tight. Actually, not even finger tight. That's about what I get when installing spark plugs with a piece of fuel line hose attached.