P0300 Misfire - Bad Headgasket?!

Always, or in particular with cold starts, then it smooths out?

Coolant level in reservoir not dropping?

I’d hold off on any more plug/coil replacements.

 

You're heading to engine dismantling You might want to hold up on firing the parts cannon many have done this here and you can read all about it The parts cannon is usually not a thing to be firing especially with this model there's too many things to go really wrong

 

Engine operation depends on many factors, fuel, air, electrics, spark plugs, etc.
It is necessary to remove the parameters of the systems operation.
You can use the OBDII scanner and programs.
Or a laptop with Techstream.
No more errors appear?

 

i'm jumping in on this thread because I've got a question for you. Mine only rumbles, and intermittently at that on start up. Last week the engine light went on, and I got misfire codes. Today there's no check engine light. I wrote a much lengthier post about 15 minutes ago. But I'm wondering why you're asking the question about start up

 

When the cars been driven, then sat overnight, the engine coolant system has pressure, and this, coupled with the overnight sit, leads to an accumulation of coolant in a combustion chamber by morning. If it’s started up in this condition the engine is struggling to get over Top Dead Center on the cylinder with (near-uncompressible) coolant.

 

so it only happens in the morning/after sitting. And not all the time. It never happens when I'm driving. From your explanation, is that confirming it's a leaky head gasket?

 

I am inclined to think so too.
You can unscrew the spark plug in the morning and look inside the cylinder.
It should have traces of a leak.

 

Yes.

 

And if you leave it long enough, you'll feel shuddering and misfiring when you take off from a stop which feels like a slipping automatic transmission. I wouldn't recommend waiting to fix that though.