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Persistent misfire cylinder 3 when hot, can't figure it out. DTC P303

J

johnwohl

New member
Hello there. I'm trying to find the cause and solution to a misfire on cylinder 3 in my girlfriends 2004 grand prix GTP. I don't know if it has anything to do with the problem but it sometimes takes a try or two to start, and sometimes you need to give it a little gas. After it starts when cold it will generally run fine until you shut it off. When you restart it after going into a store for example is when then problem usually starts. After it starts when hot you can tell the idle is very rough and when driving it seems like it is basically not firing that cylinder (duh) with very rough accelleration and vibration and lurching when stopped in gear like at a stop light. Eventually it always causes a flashing CEL and stores the DTC for a cylinder 3 misfire. If it cools down long enough it is fine again for a while. So far I've replaced all of the plugs, ignition wires, the coil for the #3 cylinder, and the injector for the #3 cylinder, and the problem persists. I have also done the lower intake manifold gasket service as it was leaking quite badly, for who knows how long before we started dating. Any help is appreciated as I am not very familiar with these engines yet and having trouble finding the solution myself.
 


I will try cleaning the MAF and throttle body to see if that helps at all. It's got 170000 or so miles and unknown service history. It actually does come up with a code for "bank 1 catalyst efficiency below threshold" once in a while, I forgot about that. I had it chalked up to a bad o2 sensor, and I didn't think it would cause a misfire in only that cylinder. It shouldn't be the injector itself as I changed that but would the wiring cause it to stick?
 
johnwohl said:
I will try cleaning the MAF and throttle body to see if that helps at all. It's got 170000 or so miles and unknown service history. It actually does come up with a code for "bank 1 catalyst efficiency below threshold" once in a while, I forgot about that. I had it chalked up to a bad o2 sensor, and I didn't think it would cause a misfire in only that cylinder. It shouldn't be the injector itself as I changed that but would the wiring cause it to stick?
Click to expand...
That code means a bad cat usually...it could also be your icm...heat kills them so possible as it gets warmer the problem worsens.
 


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