I have a 2003 blazer w/ 80,000 miles. It started idling and running rough, went to auto parts store and it coded P-300 (random cylinder misfire). went to repair shop and they said leaking fuel tube inside intake plenim. there was a charge of 103.31 for them to diagnose it for me if they didnt do the repairs. their total estimate was 1200.00. I paid them the 103.31 and took the car home with me. I undertook the repair job myself with help from the repair Manual and online video instructions and help. The fuel pressure was good and held pressure. upon removal of the intake plenim I then realized that I had the new design fuel spider where the injectors are mounted on the end of the fuel tubes. I again hooked the fuel lines and wires back up and still had good fuel pressure, no leaks and removed injectors from ports and tested them, they all sprayed upon cranking engine over and then upon designated firing order. There was nothing wrong with the fuel system,as was diagnosed. While I was right there just for peace of mind, I replaced the fuel spider with a new one. put all back together, and still the same rough idle, and eventually killed and would not start at all. I had just replaced the distributor cap and rotar about a year ago (not many miles on it). removed the distributor cap and no signs of misfire, burn or jumping fire, it looked good. I had my old cap original(GENUINE GM) that I had taken off, and just for chance replaced the newer cap with this one. got it all back together and it started on first crank and ran fine. no trouble sense. the repair job cost me about 400.00 in parts and the misdiagnosis of 103.31. take it from me with bought experience if you have same problem and P-300 code, check the fuel pressure and if good check ignition first. I heard that the recomended distributor cap is genuine GM. mine was napa but the aftermarkets dont hold up. Why? I dont know but I do Know it would have saved me alot of money and frustration. take my advise, bought experiance is the best kind. Thank You, rick A.

 

Classic example of why you should spend more time here on the forum, could have saved yourself some serious money and grief Sounds like you need a new technician, prefferably someone that is familiar with driveability issues. P0300 is almost always caused by a fault in the secondary ignition system, (Spark plugs, spark plug wires, distributor cap & rotor, and ignition coil). Aftermarket parts, (especially distributor caps) are fine when they're sitting on the shelf at the parts store. If you want "The good stuff" on your vehicle, stick with AC Delco. Aftermarket manufacturers haven't figured out how to make a cap that works, (and lasts) on a 4.3L engine.