Thread: 04 Grand Prix GTP Head gasket replaced, now multiple misfire

Compression test the cooling system and see if it holds pressure. Hook up a scan tool and see which cylinders are misfiring. Check those spark plugs plugs and look at the tips of them.

And why did you get the head gasket replaced in the first place? Because they don't go out very often, unless the car severely overheated.

Originally Posted by kman789

I had a plastic elbow going to the heater core break and as I was in the fastlane on the highway I couldn't get over and stopped before it overheated quite badly. I have had a lot of overheating problems since they fixed it and brought it back many times, it just now stopped overheating after the last time I took it they said my radiator had been clogged.

I tried to figure out which cylinder but right now its just saying multiple. I'll check the plugs tomorrow, and figure out how to compression test the cooling system as I've never done that before.

Also I feel like this is irrelevant but possibly not, they said they replaced the thermostat during the overheating fiasco and now I'm throwing a thermostat code and the engine is running rather cool compared to what it normally did.

They either threw in a 180 or 160 degree thermostat. Probably a 160* one since I've had my 180* one in for awhile with no code.

You could always pull the boot off each plug while it's running. Probably be easier right at the ignition coils themselves. If the idle doesn't change, you found a problem cylinder.