Starts and runs great but backfires under a load?

1997 CHEVROLET TAHOE
181,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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BOSTOWING
  • MEMBER
  • 1 POST
My SUV listed above starts and runs great but backfires under a load. Sometimes it will hardily drive, other times I am able to drive it fine. We recently changed the fuel pump. The MAF reads 6.3 g/s at idle then goes upward 45-50 when accelerating. All voltage is good on O2 sensors. I'll have to check timing marks, cap and rotor. Any suggestions or ideas?
Jul 17, 2023 at 8:16 AM
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JACOBANDNICKOLAS
  • ✓Experienced Mechanic
  • 110,246 POSTS
Hi,

A backfire is usually the result of either a lean or rich fuel mixture or ignition timing.

Take a look at this link:

https://www.2carpros.com/articles/engine-backfires-while-running

You mentioned the fuel pump was replaced. Did you check to make sure the fuel pressure is within the manufacturer's specifications? Have you checked for engine vacuum leaks?

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

If you pull the vacuum hose off the fuel pressure regulator, check to see if there is any evidence of fuel present. Also, with the engine running, make sure the hose has a good vacuum.

Let me know.

Joe


Jul 17, 2023 at 1:14 PM
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