I am new to this forum and have a 2002 gm van that is showing a misfire code P0300.
I have had this misfire code for some time only when I am accelerating and/or under load. Now the misfire code flashes whenever I pass 60 mph. I have never felt the engine running rough but a few times, but more recently I have. After an oil change it seems to stop the misfire until I am within the last 1000 miles of the 3000 between changes. I switched to full synthetic oil which seemed to stop it from misfiring the first 2 changes, but the last 1000 miles it has flashed every time I pass 60. I have had a complete tune-up. When I had it checked at a GM dealer early on (out of warranty) they wanted to remove the cylinder and check for binding in the valve. Then possibly machining the valve stem and hone the guide. Very expensive. I was advised that if it wasn't running rough and it only was happening infrequently not to worry about it.
Any ideas?

 

have fuel pressure checked. v8 should be 60psi. if it's not 60psi, replace fuel filter, recheck. still not 60psi, fuel pump may need replaced, or fuel pressure regulator. also, change air filter if needed.