I Have this 2004 Silverado 1500 ext. cab long bed, 5.7L with the P0300 (engine misfire detected) code. I've replaced spark plug wires and plugs, cleaned the throttle body, and replaced all O2 sensors. with no luck, the engine idles a little rough but no issues with acceleration. I have used the snap D10 triton diagnostic tool to find that cylinders 3, 4 and partially 5 have misfires. I have not checked fuel rail pressure or coils yet. is there any common issue or tips to help diagnose this issue? also does anyone have a diagram of the fuel lines. I'm not sure if this truck has the "lifetime" fuel filter inside the tank or the inline filter along the frame rail. I can't seem to find it, so I am assuming it is with the fuel sender. but I have also have not seen anything like what this truck has with the way GM has routed the fuel lines. in between the tank and the engine there are fuel lines leading to this reservoir that is shielded with sheet metal over it directly underneath the bed. any information would help immensely, thank you.

 

I just fixed a miss on number 1 cylinder- it was the injector connector. I used a noid light to check for power at the connector and soldered on a used one from a parts truck

 

The fuel filter should be between the fuel tank and the engine. It should not be incorporated into the fuel pump assembly.

Also, the 5.7 L V8 engine was not offered in the 2004 Silverados. Is the engine a 5.3 L V8?

What spark plugs did you use? If you did not use ACDelco 41-962 plugs then you are asking for misfires. These engines are darn picky about what spark plugs go into them. Also, if you purchased any spark plugs from Amazon or ebay, the likelihood you received counterfeit spark plugs is extremely high! Want to save money on spark plugs? Purchase them from rockauto.com (very safe).

Double check the electrical connections to the fuel injectors and ignition coils for the cylinders showing misfire counts.