Expert: Skip Bollinger - 11/7/2010

Question
HI, I have a 95 chevy 2500 with a 5.7 liter v8 Recently my truck started losing power but only while driving down the road. While in gear when I step on the gas it bogs down and starts backfiring, but when in park and sitting still I can rev it up to red lining it and doesn't skip or miss. What could this be. my first guess was spark plugs or timing. could you please give me some insight. Thank you
JD

Answer
Jonathan,
  First of all, revving your engine to red line in Neutral or Park is NOT A GOOD IDEA, Okay?? Now to your question..
  Secondly, I'm a Parts Pro, NOT a Tech.. If my suggestions don't help, please don't slam my chops for being stupid, agree?? I will offer these thoughts for your consideration. Here we go..
  The first thing that comes to mind when you say, while in gear, it begins to backfire. You don't say if it is backfiring through the Intake or Exhaust. IF it is the intake, this usually is caused by a "lean" condition. It could be a cracked vacuum hose, clogged fuel filter, insufficient fuel pressure or a bad intake manifold gasket.
  What you might try is taking the air intake hose off the air intake. Start the truck and put a paper towel over the air intake to "choke" off the air. What does the idle do? Does it come up or does the engine stumble? IF it comes up, look at the items I have listed above. If it doesn't, I'm not sure what to look at. It could be "spark arcking". In a dark garage or at night, get a "plant mister" and start the truck. Spray the spark plug wires. Do you see a "light show" (arcking) from the wires? If so, replace them.
  Now if the backfiring is from the exhaust, I'd suspect the the wires. It could be timing but that would happen with the vehicle not being under load or in gear.
  I know this is a lot to ingest but I hope it helps give you some direction. Good luck.
Skip