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Popping out the exhaust, rough idle

Tiny
GMC3500HELP!
  • MEMBER
  • 2000 GMC C3500
  • 7.4L
  • 168,000 MILES
Hi,
My GMC Sierra C3500 with a 454 engine (non-dually) is popping out the exhaust when I drive slowly, especially when I let off the gas and start to slow down, running terribly, low on power, sounds bad too.
Also started throwing po174 code (but this happened soon after I clean the mass airflow sensor, so I'm not sure if I triggered it or if it's the truck triggering it).
These symptoms occurred suddenly a couple weeks ago after a weird thing happened : was cruising at like 65 mph, truck running just fine. When I noticed smoke rising up in the rear. I pull over and there is a smoldering board laying on the exhaust between the cab and bed, and then there's something stuck to my exhaust system underneath the truck on fire! I'm assuming what was stuck to the exhaust system was some kind of trash or something? A police officer nearby ran over and hit it with a fire extinguisher.
So I get back in the truck, crank it up and take off and the symptoms start. I don't know if the fire was a weird coincidence or what?

One shop said I have a burnt valve and a mystery vacuum leak, another local shop said that they are "90% sure" it has a " burnt valves", but after reading your article about back-firing, I am not so sure.
Second shop claimed they did a pressure test, but I don't believe them.
I don't like their "90 %" odds because the burnt valve repair is going to cost me like $1,500.00 - $2,000.00.
However, they might be telling the truth because I did the dollar bill trick in front of the exhaust pipe and the dollar bill definitely flaps in and out.

I have changed the plugs and wires, got a new muffler, cleaned MAF sensor, new kn filter, and deleted the rear catalytic converter because one mechanic said that it might be clogged (previous owner deleted the rear O2 sensors, so it didn't really bother anything).
What to do?
Thank you!
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Sunday, August 4th, 2019 AT 4:46 PM

10 Replies

Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

Okay, to see if you have a burnt valve or not is pretty simple, do a compression test. Here is a link for you to go to below:

https://www.2carpros.com/articles/how-to-test-engine-compression

Please go through this guide and get back to us with what you are able to find out. If your compression test comes back okay, we will have a factory diagnostic troubleshooting guide for you for Direct Trouble Code(DTC) P0174 that will pinpoint what is going on with your vehicle for you. We want to do one thing at a time, and take a systematic approach to fixing your vehicle.

Thanks,
Alex
2CarPros
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Monday, August 5th, 2019 AT 5:40 AM
Tiny
GMC3500HELP!
  • MEMBER
  • 6 POSTS
Just had a compression test, all cylinders in the 150s ("good" I was told). Said my fuel pressure is low, maybe it is what is causing it to run lean. Said the lean condition could possibly cause these symptoms. I am hopeful.
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Monday, August 5th, 2019 AT 12:43 PM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Yes, low fuel pressure will indeed cause a lean condition, in fact it is pretty much the definition of a lean condition. Pretty sure that getting that fixed will take care of the problem. Regardless I have included in the diagrams down below the factory diagnostic troubleshooting guide for Direct Trouble Code(DTC) P0174, merely for reference for you. Please go through this guide, at your leisure, and keep us informed as to how everything is progressing.

Thanks,
Alex
2CarPros
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Monday, August 5th, 2019 AT 11:31 PM
Tiny
GMC3500HELP!
  • MEMBER
  • 6 POSTS
Didn't fix it. Think it is done, can't afford to just keep throwing parts at it. Thanks though.
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Tuesday, August 6th, 2019 AT 6:36 AM
Tiny
GMC3500HELP!
  • MEMBER
  • 6 POSTS
P.S
This all started when some trash stuck to my exhaust and caught on fire. Perhaps it's a coincidence but I am not so sure. Thinking there's got to be a link between that event and these symptoms, but can't figure it out.
Installed new muffler. The people that installed the muffler said they didn't see anything out of place. I don't know.
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Tuesday, August 6th, 2019 AT 6:40 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Check the wiring going to the upstream O2 sensor. By upstream I mean the O2 Sensor that is closest to the engine on the side the trash was burning on. Please get back to us with what you are able to find out.

Thanks,
Alex
2CarPros
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Tuesday, August 6th, 2019 AT 6:45 AM
Tiny
GMC3500HELP!
  • MEMBER
  • 6 POSTS
Have it in a shop, they claim to have checked *everything* and that its "all good" except my fuel pressure is still low by like "10+ psi" and that they are confident that this has to be it. I could probably replace the fuel pump myself but unfortunately don't have the time, - having them do it is somewhat expensive but I have no choice.
I hope this is it, but if you guys disagree feel free to let me know.
It kinda worries me, didn't know/think that 10psi low fuel pressure would cause my truck to pop and run badly.
Thank you
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Wednesday, August 7th, 2019 AT 6:48 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Have them check the upstream O2 sensor and wiring harness going to the upstream O2 sensor on the side that you had the trash burning on. I have seen these wires melt together just from them touching the exhaust and it will cause these problems as well. Something else to consider, if it was low fuel pressure causing this, your vehicle should also display DTC P0171 which is Fuel Trim Too Lean/Bank 1, not just DTC P0174 which is Fuel Trim Too Lean/Bank 2. The lack of fuel pressure would be spread across the fuel rail, effecting both banks, not just one. Please keep us informed as to how things progress.

Thanks,
Alex
2CarPros
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Wednesday, August 7th, 2019 AT 11:29 PM
Tiny
GMC3500HELP!
  • MEMBER
  • 6 POSTS
Shop claimed replacing the pump stopped the popping but still running rough, " misfiring". I might be getting taken for a ride at this point, hope not.
I suspect that you might be right, I mean the fire had to do something under there and what else could it be? Thank you
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Thursday, August 8th, 2019 AT 3:50 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Ask the shop if the DTC has gone away? Please keep us informed.

Thanks,
Alex
2Carpros
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Thursday, August 8th, 2019 AT 6:10 AM

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