Have a P0304 on my 2006 Pontiac G6 2.4 (244K)

I have switched plugs, coils & injectors. Not seeing any leaks in the intake and hosed it down with brakleen hoping to hear that get sucked up but I heard no change in idle. My code reader registers misfires only at idle and it'* not enough to be every single time the cylinder is supposed to fire. Maybe 10 misfires every 30 seconds. When above around 1200 rpm I get no misfires. When I pull the power to the coil, the misfire is very noticeable so it must be working partially.

On a long drive (300ish miles), mostly highway, I had 423 Cylinder 4 misfires, 11 cylinder 3 misfires, 2 cylinder 2 misfires and 0 cylinder 1 misfires.

I've read elsewhere it'* the head or exhaust port (NOOO!!! haha).

Gotten by emission by driving it on the highway for long periods then going to an emission place near the highway.

Any help diagnosing would be greatly appreciated.

Do a cylinder compression test on all of them with the engine cold, then hot.

How is your coolant level? Oil level?

I'll do a compression test across all cylinders. Cylinder 4 got up to 90psi in a few cranks, don't remember whether it was hot or cold. It only turned over a couple times and I figured that was sufficient but I'll do the hot/cold you suggested. The coolant level is slowly dropping (add about 1 cup a month) and it burns a little oil so I add about a quart between changes.

Oh, is it safe to remove plugs on a hot engine with aluminum head?

Yes it is.

Not sure when I'll get a chance to do a cold test, hot/warm compression is cyl1: 120 cyl2: 120 cyl3: 120 cly4:105. That'* in cylinder order not firing order, fyi. Dun, dun, DUN! So I guess that'* kinda (though not terribly) low compression. Do you think a heavier weight oil could improve it marginally? Running 5w30.

Oil won't have anything to do with how your head gasket seals. If you are actually consuming coolant, and the misfires coincide with the low cylinder compression, then I would say you probably have a bad head gasket. You could get a combustion leak checker that you install on top of your radiator to find out for sure.

I guess, it'* so little in the way of coolant loss (and my plug isn't steam cleaned) I was more worried about ring/cylinder damage which is why I asked about oil. I'll do a coolant test and report back. I appreciate your time Mike!

We do our best to help.

Got my test kit in, since my only access is the expansion tank, I stuck it on there and let it run for about 30 min, had some bubbles come through and pumped it every 2-5 min. Solution in the tester didn't change color. Likely cylinder/ring issues?