Hey everyone, I’m new here so forgive me if this question has already been answered but I’m in serious need of some help. can’t really afford to take it to a shop right now but I have a 5 speed 4.3 liter vortec ‘97 Chevy Cheyenne c1500 I just recently bought. It ran fine for about 2 weeks then started blowing white/grayish smoke out if the tailpipe & smelt of raw gas STRONG. service engine soon light came on & started blinking I plugged in my code reader and got a few codes, p0300 being one, but the “primary fix” said mass air flow sensor. I replace that with a brand new one & it helped some, but not much. I clear codes & hook it up again & now I’m just getting the random misfire code (P0300) I then replace all spark plugs (one of them had oil & gas on the tip) & just today replaced all spark plug wires. after doing that it made it 20x worse. Still missing, still blowing smoke & dripping gas out of exhaust, hesitating & will barely pull. I can put it in neutral and it will come to a dead stop, almost as if the E-brake is on. still sounds as if a plug is spattering. I don’t know what else to do. if anyone has any suggestions, please comment below.

 

I believe that at least one of your cylinders has something VERY wrong with it, or you have a blown head gasket (less likely if the motor slows down like an E-brake). Could easily be a valve problem as well.

I would buy or rent a compression tester and check all your cylinders compression. Ideally, they should be within 15 percent of each other. If not something is seriously wrong with the motor and it will need to be repaired.

If you want a simple down and dirty way to see if something is amiss find a vacuum port and see if the vacuum gauge fluctuates, which it almost certainly will if anything is wrong with your engine running as bad as you describe. Not as easy to find a small vac-port as it once was with carburation, but adapters from a larger to a smaller vacuum source are pretty easy to find at an automotive parts store. Frankly, I would not waste that time and money and would run the compression test on the cylinders.