Just wanted to add my experience for others searching this P0300 random misfire code issue. 1998 Astro Cargo van with 155k.
My van ran pretty good most of the time it would really only give me the code when going uphill at freeway cruising speeds. The SES light would come on and when the van really started acting bad SES would blink. Van would start spuddering and lose power. At first it was only when carrying a load but after awhile it would do it everytime I went up this hill on the freeway even without a load. One time it was so bad I had to pull over and shut down the van and give it a minute before firing it back up and it ran fine.
My mechanic couldnt narrow it down so I finally brought it to the dealer. They diagnosed it for about 5 hours and came to the conclusion that it needed a valve job. Quoted me something like $2800 for that or $6500 to swap out the motor for a new. Its a work van and I didnt have the liberty of not having a van for even a week so I went out and bought a low mileage 03 instead.
A few things I had done prior to this was swap out the plugs wires cap and rotor, all AC Delco. Cleaned up the Map sensor, repaired a faulty gasket, cant remember if it was head gasket or intake. It was basically patched up somehow with a technique of running some chemical thru the cooling system and flushing it a dozen times until it hardened up where the repair was needed. It failed a leakdown test prior and passed afterwords so I guess it held up. Still gave me a random misfire code after that. Also put in a new distributor gear assembly even though on the old one the teeth looked fine. None of that was a fix.
Thats my story I appreciate all the info on this site and so wanted to share my experience with others trying to track down this issue. Hopefully your story has a happier ending.
My van ran pretty good most of the time it would really only give me the code when going uphill at freeway cruising speeds. The SES light would come on and when the van really started acting bad SES would blink. Van would start spuddering and lose power. At first it was only when carrying a load but after awhile it would do it everytime I went up this hill on the freeway even without a load. One time it was so bad I had to pull over and shut down the van and give it a minute before firing it back up and it ran fine.
My mechanic couldnt narrow it down so I finally brought it to the dealer. They diagnosed it for about 5 hours and came to the conclusion that it needed a valve job. Quoted me something like $2800 for that or $6500 to swap out the motor for a new. Its a work van and I didnt have the liberty of not having a van for even a week so I went out and bought a low mileage 03 instead.
A few things I had done prior to this was swap out the plugs wires cap and rotor, all AC Delco. Cleaned up the Map sensor, repaired a faulty gasket, cant remember if it was head gasket or intake. It was basically patched up somehow with a technique of running some chemical thru the cooling system and flushing it a dozen times until it hardened up where the repair was needed. It failed a leakdown test prior and passed afterwords so I guess it held up. Still gave me a random misfire code after that. Also put in a new distributor gear assembly even though on the old one the teeth looked fine. None of that was a fix.
Thats my story I appreciate all the info on this site and so wanted to share my experience with others trying to track down this issue. Hopefully your story has a happier ending.