P0300 misfire

[tcamaross]

hi guys, i'm new to this forum and wondering if anyone can help. i have a 2002 GMC Yukon SLT with 5.3 vortec flex fuel. 174k miles. i'm getting a p0300 code and cant seem to find out the issue. ran autotap on it and found misfires randomly at idle, and then more prominent at 1500-2000 rpm. picking up misfires mainly on cylinders 1 and 5 so i tried moving coil packs (swapped 1-3 and 5-6). no luck. then thought there may be a vacuum leak, sprayed carb cleaner around the intake manifold and no change in idle. O2s pre cat seem to be oscilating high-low mv consistently and from what i've found online that is good. post cat O2s dont come up oscilating like the pre cats do, but they do move with engine speed. tried disconnecting them too and there is still a misfire. checked the vacuum line on the fuel pressure regulator and there was no fuel, just good vacuum, so thats ok. MAF disconnected and then the computer doesnt report misfires but the engine still feels like it has the misfire between 1500-2000. i guess i could try cleaning the MAF, but how can i do that safely without causing more problems if this isnt the culprit? only thing left is checking fuel pressure (but i've got the misfires mainly on bank 1, believe bank 2 is being caused by bank 1 as they are much lower in the readings), injectors. i changed the fuel filter within the past year so i dont think thats it. if the battery is a little weak, could that cause this?? also, if i take the rpm up to 4500-5000 the voltage gauge drops, below 12. what are the chances this could just be bad gas?

thanks

 

[clkelley]

Welcome to the neighborhood!

I know this sounds extremely simple, but check those misfiring cylinder plugs, make sure they are still fully seated in the head and that the wires are fully seated on the plugs and that the plugs aren't burnt, etc. If the plugs happen to be new(er), take them out and check the gapping on them.

 

[tcamaross]

thanks for the quick response. i pulled #1 and #3 and they looked good, gap was right on too 0.060 if i recall correctly. i can try swapping wires and plugs between banks. i've never changed the o2 sensors, any chance even though the readings look ok they could be the problem?

 

[clkelley]

They could be, they don't have to throw a code to cause a problem.

 

[tonymac617]

If you still wanted to try to clean the MAF there is a specific cleaner to do that and they sell them in Autoparts stores. Also, did you say you checked your Fuel injector pulses? Or just the bank 1 bank 2 fuel trims?

 

[clkelley]

And one last thing, did you clean your throttle body? Mine was nasty and although it wasn't a miss per say, but it did have a stumbling idle that wasn't smooth.

 

[Grebbler]

2 more things to consider. That motor and year has an issue with intake manifold gaskets failing causing lean misfires. Use Felpro Pro problem solver gasket kit.

Spark plug gaps were changed from .60 to .40. Only use AC spark plugs to avoid misfires and other problems. These motors are picky in that regard. You might get by with others but why invite trouble.