Thread: P0303 Code. Help!!!! Misfire!!!

Thanks in advance any and everyone for your help on this. I am not a mechanic but very mechanically inclined and good with tools. So my wife had a blown headgasket on her 03 GrandPrix, Vin K, around 125k miles. So I replaced it, put the car back together and now getting this dreaded code thrown (P0303). Cylinder 3 misfire. The car slightly idles like there is a misfire, but still runs better than it did or course. I've done all suggested repairs and had the heads checked out before reinstalling them. I took the sparkplug back off to, but it looks fine with no substance build up. I drove the car for around 10 minutes and let it idle for around 40min. The car sat for about 5 months. Fuel is around 1/4 tank. One dreaded mistake I did make was put most of the car back together and realized I didn't have enough room to insert one of the bolts on right motor mount. But it still has two. Would this be part of the issue? Does this hole feed all the way back into the Left head? I will fix this next weekend. But not sure where to start here. No leaks around LIM gasket, or heater L-hoses. Also, should I add a octane boost to my fuel, just because it's been sitting?

I would check the plug wire. Maybe when you removed it it got damaged. Beyond that you can always swap coils and see if the miss stays on cyl 3 or changes to another cylinder. If it stays then you know the issue is with 3 still.

And no, "octane boost" will not do anything other than lighten your wallet. Sitting for 5 months isn't enough time to make a difference. If anything, maybe throw a bottle of Heet in it and then fill it up with fresh fuel. My classic car sits longer than that and fires up/runs just fine on the old fuel.

Thanks for the response FordMan. I noticed I put to much oil in there as well. >1.5 qts over. Not sure what I was thinking, but will remove that today. Also, this is a new set of wires and plugs, so I will try to swap cylinder 3 wire with 2 and see what happens. I will double check the coil as well. Dont have a extra one laying around. I drove it to the store, and it ran pretty good, but idles bad. I won't drive it anymore until I atleast get some oil out of there.

cdubb- I forgot to ask, and I know this sounds stupid and you didn't mention it but did you verify that you have the plug wires in the correct firing order? I ask because I've seen guys get 2 of them in the wrong spot and drive fine but idle wierd. And yes, they swore they had it right, lol... Until they checked it again.

Ahhh... Duhhh. "Coil Pack" . Okay. Will do that in a few. Fired order is correct. rear 246 and front 135. I drained some oil and cleaned off the maf sensor and throttle control valve. It did help with the low idle, but still have the code. So will test coils in about 15 min and return.

Ok. Coil Pack switched. Still Cylinder 3 misfire. So it is now confirmed the coil packs are either good or I have two bad ones lol. Definitely Cylinder 3 though.

I have not swapped the injector yet. I took it to a local mechanic and I paid the a$$hole $30 to tell me "just get a new motor". He did a compression test and there is no compression on this cylinder. I'm assuming since I had him clean up the heads and I didn't touch the valves, that the valve must be sticking and not closing all the way. Does this sound right so far??? He said the injector and coils is fine. I may just go buy a reman. head for the left side and redo just the left side. At least I don't have to take off the rear exhaust manifold. That was a Bleep... On another note, I drove it to the mechanic today. Do you think I will mess something up doing this? Lesson for the day. "DO IT RIGHT THE FIRST TIME... TIME IS MONEY"

Will do Scotty. I'll double check everything under the valve cover myself. Push rods are straight. I did the rolling and cleaning them myself. Will check all rocker bolts, because I did reuse them. Man you guys are great. You just can't beat hands-on experience these days. I will update you guys soon as I have time to tear it back down. Will probably be thursday, saturday, or sunday. Thanks much guys. I don't see any point in buying a new engine if i'm capable of fixing this one. As it stands the car is not overheating anymore. Thanks again.