Rough running backfire

Tiny
DAVIDLWELLS
  • MEMBER
  • 1998 CHEVROLET TAHOE
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 178,000 MILES
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.
Saturday, August 15th, 2020 AT 5:07 AM

5 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
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
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Saturday, August 15th, 2020 AT 7:05 AM
Tiny
DAVIDLWELLS
  • MEMBER
  • 4 POSTS
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.
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Saturday, August 15th, 2020 AT 8:28 AM
Tiny
ASEMASTER6371
  • 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
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Saturday, August 15th, 2020 AT 3:55 PM
Tiny
DAVIDLWELLS
  • MEMBER
  • 4 POSTS
This one sets with a scanner. It sets at zero. It has been set.
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Saturday, August 15th, 2020 AT 5:39 PM
Tiny
ASEMASTER6371
  • 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
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Sunday, August 16th, 2020 AT 4:42 AM

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