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Runs rough and no power under load

Tiny
SULLYMECH
  • MEMBER
  • 2004 CHEVROLET SILVERADO
  • 120,000 MILES
A coworker brought me his 04 silverado with the complaint of no power and a trouble code, which he said was from bad o2 sensors. I borrowed a motis and he got new 02's swapped them out, I changed the o2's and cleared the codes for emissions and now I have a miss fire on cyl 4 and 6. I've replaced plugs, swapped coils, tested the map, maf, iac, checked injectors and intake for leaks. The only test I have yet to do is a compression test. The truck still idles rough and falls on its face under any type of a load. Is there anything that I missed or overlooked. Thanks
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Monday, May 6th, 2013 AT 4:01 PM

4 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
What were the codes? They never identify bad parts just failed systems.

Compression test is first to do. Check fuel pressure. Check back pressure on the cat converter as well.

Roy
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Monday, May 6th, 2013 AT 4:15 PM
Tiny
SULLYMECH
  • MEMBER
  • 3 POSTS
It was a p0300 and I checked the motis to find which cyl had the misfire. Fuel pressure is 58 psi. I initially thought the problem might be the cats because of the power loss but I didn't think it would cause a misfire
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Monday, May 6th, 2013 AT 4:30 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Fuel pressure is border line.

Cat converter will cause a miss fire. Check the back pressure to confirm.

Roy
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Monday, May 6th, 2013 AT 4:35 PM
Tiny
SULLYMECH
  • MEMBER
  • 3 POSTS
I wanted to let you know we solved the problem. The left cat had 3 psi of back pressure and the right pegged my gauge as soon as we started the truck. I temporarily hollowed them out and all power returned no misfires and ill be installing a new ypipe next month when the owner gets it in
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Friday, May 10th, 2013 AT 6:08 PM

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