Hi, I have been searching for a while now, and decided to register and ask now. I have a 99 GM and I just got a code that read p0304, and this is misfire in cylinder 4. From my research I see it could be a multitude of things, but first I need to see if it's the coil/spark plug.

I'm going to get a new spark plug, and switch the coil to another cylinder to see if it is the coil...

Is there any advice anyone can give me for this? I hope it's just the coil/plug, but I could be in for a surprise.

 

could be a misfire due to moisture in there where the boot meets the plug.

 

my advice is don't switch the wires between cylinders. Check the connections to the coil and the spark wires - make sure everything is connected properly and the boots are shoved fully down onto the spark plugs. If you have over 100K miles on it, you may want to replace the wires anyhow. Pull the plug and inspect it for corrosion, fouling, physical damage. If it got oil caked all over it, you may need some hotter plugs to get a spark because you've got oil leaking into that cylinder. if you have corrosion, you've got water/coolant collecting in that cylinder and need to find out why. If there is simply hard black carbon coating the plug, it should just be replaced and some fuel injector cleaner run through the system. the injector may be leaking and/or not spraying properly. I know I should probably put some new wires on my MGM (may do that this weekend when I change the oil). If the plug's ceramic insulator is brown and the electrode is rounded like a worn eraser, then it's just overly worn and needed to be changed a good while ago.

If your car has the dual coil packs like mine, you could swap the coil packs and see if the misfire changes to another cylinder. if that's the case, then you have one bad coil pack and should just replace that.

 

Thanks for the replies. I think I might have figured out why it fried. I took off the coil, and I see a hose with coolant that seems to have a slow leak. The leak is going right into the coil/plug. Is this a pretty common problem, or do you think it was something else?

 

that could be the problem. replace the hose and clean out the plug/coil and see if that fixes the issue. If not, you may need a new coil.

 

So I changed the coil/plug, and the check engine went off. The only thing is there was a lot of oil in the plug well. What does this mean? I figure it's some sort of gasket leaking...

 

sounds like you possibly need the valve cover gasket kit...

 

I'll second what 87_crown_Vic said. It's probably the ring seal/gasket where the cover meats the head. that means you'll need to replace the valve cover gasket and all 4 ring seals on that side (along with any other seal that gets separated in the process - never worked on a 4.6 valve cover before).