Rough running backfire

1998 CHEVROLET TAHOE
178,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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DAVIDLWELLS
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  • 4 POSTS
This SUV has me stumped. Bought it used had to make some repairs. Have changed motors in it. The old motor in it when was running rough sounded like wrist pin was knocking. Motor came out of 1997 pickup ran great. Since put motor in have replaced fuel pump temperature sensors all 4 O2 sensors. New spark plugs and wires. New distributor cap and rotor. Tried different PCM. When running rough will hardly idle when going up a hill have to pat the gas other wise will bog out it will not move.
Aug 14, 2020 at 10:07 PM
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ASEMASTER6371
  • ✓Experienced Mechanic
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Good morning,

Are there any codes set?

Have you checked the compression on the donor motor to be sure there are no issues?

https://www.2carpros.com/articles/engine-has-low-power-output

Roy
Aug 15, 2020 at 12:05 AM
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DAVIDLWELLS
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Did not do compression test. Engine ran great when was in truck. 2 different engines in this SUV and both has this issue. Is it possible that wear on distributor can cause this issue? Can't say for sure which one was put back in it when done engine change.
Aug 15, 2020 at 1:28 AM
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ASEMASTER6371
  • ✓Experienced Mechanic
  • 52,796 POSTS
You need to do the compression test.

As far as the timing, do you have a timing light to give me the timing?

How did you set the timing?

Roy
Aug 15, 2020 at 8:55 AM
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DAVIDLWELLS
  • MEMBER
  • 4 POSTS
This one sets with a scanner. It sets at zero. It has been set.
Aug 15, 2020 at 10:39 AM
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ASEMASTER6371
  • ✓Experienced Mechanic
  • 52,796 POSTS
Okay, did you check the fuel pressure to be sure the fuel pump is providing the correct amount of fuel?

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

https://www.2carpros.com/articles/engine-has-low-power-output

That would be my next step. You can rent a tester at a parts store to verify the pressure.

Roy
Aug 15, 2020 at 9:42 PM
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