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Posted: 12/13/2013 4:38:53 AM EDT
I'm looking at a used GMC sierra with the 4.8 (90,000 miles) that has had the catalytic converters removed, for whatever reason.  The truck runs fine except for a backfire (sometimes, more prevelant when the engine is cold) when accellerating.  If I buy the truck I'l replace them regardless.  Just wondering if anyone thought this might cure the random backfiring.
Link Posted: 12/13/2013 4:41:02 AM EDT
[Last Edit: 12/13/2013 4:44:19 AM EDT by RDTCU]
Originally Posted By oxfordethan:
I'm looking at a used GMC sierra with the 4.8 (90,000 miles) that has had the catalytic converters removed, for whatever reason.  The truck runs fine except for a backfire (sometimes, more prevelant when the engine is cold) when accellerating.  If I buy the truck I'l replace them regardless.  Just wondering if anyone thought this might cure the random backfiring.
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That's not backfiring, it's from unburned fuel vapors re-igniting, and yes, removing the cats can make it more likely to happen.
Probably more prevalent when cold due to the cold startup enrichment, which dumps more fuel until the engine is warm.

What is the rest of the exhaust system like?  The shorter it is without cats, the worse it can be.

If it's truly backfiring, as in blowing a fireball out of the intake, then I wouldn't buy that truck
Link Posted: 12/13/2013 4:52:03 AM EDT
The guy just cut the cats out and welded in straight pipe.  The rest is stock.  It's more like a popping noise under your feet wheen driving.  Like it's popping in the exhaust where the cats should be.
Link Posted: 12/13/2013 4:56:02 AM EDT
I removed my cats on the Ford f150 I used to own. It didn't make those noises and in fact, it made the exhaust sound badass. It got better mpg as well.
Link Posted: 12/13/2013 4:58:31 AM EDT
FPNI
I know it doesn't indicate any performance but I love a good cackle on decel.
Link Posted: 12/13/2013 4:58:33 AM EDT
[Last Edit: 12/13/2013 4:59:46 AM EDT by RDTCU]
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Originally Posted By oxfordethan:
The guy just cut the cats out and welded in straight pipe.  The rest is stock.  It's more like a popping noise under your feet wheen driving.  Like it's popping in the exhaust where the cats should be.
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Check the welds for leaks, if it's obnoxious, it's probably because it's getting some fresh air from somewhere, which mixes with the hot fuel vapors and re-ignites.

Sometimes it's just a resonance thing when you get burning vapors in the exhaust.
Link Posted: 12/13/2013 5:01:34 AM EDT
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Originally Posted By Alpha82:
FPNI
I know it doesn't indicate any performance but I love a good cackle on decel.
View Quote


There's a difference between an engine braking "burble burble" and a "burble POW burble POW POW"

The latter is actually indicative of being lean.
Link Posted: 12/13/2013 5:17:29 AM EDT
Thanks, guys.  I've looked over the rest of the truck and can't find anything else wrong.  I'm hoping I can used this as a bargaining chip to get the price down.
Link Posted: 12/13/2013 5:38:07 AM EDT
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Originally Posted By RDTCU:


There's a difference between an engine braking "burble burble" and a "burble POW burble POW POW"

The latter is actually indicative of being lean.
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Originally Posted By RDTCU:
Originally Posted By Alpha82:
FPNI
I know it doesn't indicate any performance but I love a good cackle on decel.


There's a difference between an engine braking "burble burble" and a "burble POW burble POW POW"

The latter is actually indicative of being lean.


I'm definitely talking popping LOUD and sort of irregular, my old '87 5.7TBI would flash off out the pipes when it did it and the pops could wake a whole neighborhood. It ran on the rich side.
My 07 5.4 only does it during the few seconds right after start-up, never known a vehicle running right to be lean right after start-up.
Link Posted: 12/13/2013 6:02:57 AM EDT
[Last Edit: 12/13/2013 6:05:28 AM EDT by RDTCU]
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Originally Posted By Alpha82:


I'm definitely talking popping LOUD and sort of irregular, my old '87 5.7TBI would flash off out the pipes when it did it and the pops could wake a whole neighborhood. It ran on the rich side.
My 07 5.4 only does it during the few seconds right after start-up, never known a vehicle running right to be lean right after start-up.
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Originally Posted By Alpha82:
Originally Posted By RDTCU:
Originally Posted By Alpha82:
FPNI
I know it doesn't indicate any performance but I love a good cackle on decel.


There's a difference between an engine braking "burble burble" and a "burble POW burble POW POW"

The latter is actually indicative of being lean.


I'm definitely talking popping LOUD and sort of irregular, my old '87 5.7TBI would flash off out the pipes when it did it and the pops could wake a whole neighborhood. It ran on the rich side.
My 07 5.4 only does it during the few seconds right after start-up, never known a vehicle running right to be lean right after start-up.


The loud pops on decel are lean, it doesn't matter if it runs rich under normal operation. What kind of FI was it running?  Speed-density, MAP or MAF?
I'm guessing MAP.
Having tuned a few FI bikes and autos, and written maps from scratch on a Motec, decel pops = lean
If you don't have any over-run tuned in, when you lift, the fuel shuts off.  Exhaust is normally too rich to ignite, but if you start pumping a small amount of fresh air, with no fuel into it (due to your idle set screw or IAC valve) it will periodically create an ignitable mixture, and that's the POP.

The only way to eliminate it is to dial in a little over-run, because richening up your idle mixture will just make it run stupid-rich when you're actually at idle RPM.

As for the 07, what's your exhaust system like?  They run crazy rich on startup, and the unburned fuel could be mixing with the fresh air still in the exhaust until it hits the magic ratio and lights off.
Link Posted: 12/13/2013 6:09:53 AM EDT
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Originally Posted By oxfordethan:
Thanks, guys.  I've looked over the rest of the truck and can't find anything else wrong.  I'm hoping I can used this as a bargaining chip to get the price down.
View Quote


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.
Link Posted: 12/13/2013 6:19:02 AM EDT
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Originally Posted By P400:


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.
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Originally Posted By P400:
Originally Posted By oxfordethan:
Thanks, guys.  I've looked over the rest of the truck and can't find anything else wrong.  I'm hoping I can used this as a bargaining chip to get the price down.


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.


Haha, oh yeah, that too...

If he DIDN'T dummy the rear O2's out, it could explain the lean decel, but it would probably have thrown a code by now...
Link Posted: 12/13/2013 6:19:20 AM EDT
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Originally Posted By P400:


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.
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Originally Posted By P400:
Originally Posted By oxfordethan:
Thanks, guys.  I've looked over the rest of the truck and can't find anything else wrong.  I'm hoping I can used this as a bargaining chip to get the price down.


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.


Planning on this.  I hoping he will give me the name of the guy that did "the work" so I can get it reversed.  It's not throwing any codes right now.

Link Posted: 12/13/2013 6:23:31 AM EDT
not sure about your truck but my rotary car w/o a cat shoots big flames from my exhaust tips and i love it. i always make a point to down shift when im passing a prius
Link Posted: 12/13/2013 6:40:55 AM EDT
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Originally Posted By P400:


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.
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Originally Posted By P400:
Originally Posted By oxfordethan:
Thanks, guys.  I've looked over the rest of the truck and can't find anything else wrong.  I'm hoping I can used this as a bargaining chip to get the price down.


You definitely should. Also ask him if he installed 02 simulators as well or made some other mod to trick the PCM into thinking that the cat is still there. You would need to reverse such a mod if you were to replace the cat.


+1

Make sure you make that guy pay for doing something like this on a truck.
No one else is going to buy it from him running like that.

Link Posted: 12/13/2013 7:08:11 AM EDT
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Originally Posted By endofdays:
not sure about your truck but my rotary car w/o a cat shoots big flames from my exhaust tips and i love it. i always make a point to down shift when im passing a prius
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That's just a rotary thing  Crazy hot EGT's
Link Posted: 12/13/2013 8:00:50 AM EDT
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Make sure you make that guy pay for doing something like this on a truck.
No one else is going to buy it from him running like that.

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Keep in mind I'm in Mississippi...a mod like this can actually drive the price up.
Link Posted: 12/13/2013 10:51:58 AM EDT
It shouldn't be backfiring, what you're hearing is probably some pop on decel. Cats, and anything inline on the exhuast for that matter, quiet this down; with it removed, it becomes louder.

Sorry to sound simple
Link Posted: 12/13/2013 12:26:12 PM EDT
my bet is on the engine burns oil and loaded up the cats. Too expensive to repair the engine and replace the cats.
off comes the clogged cats,"look at the underbody and lift the floormats for signs of scorched paint/ melted rugs.
Thats a Federal and State offense to pull out the cats on a on-road vehicle. Don't expect the shop to own up to it.
Big No-No and puts his repair shop license on the chopping block..
Race Cars and Off road vehicles are a different story.
I'm voting you walk away, the tranny is probably ready to puke as well.
Link Posted: 12/13/2013 2:27:53 PM EDT
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Originally Posted By RDTCU:

The loud pops on decel are lean, it doesn't matter if it runs rich under normal operation. What kind of FI was it running?  Speed-density, MAP or MAF?
I'm guessing MAP.
Having tuned a few FI bikes and autos, and written maps from scratch on a Motec, decel pops = lean
If you don't have any over-run tuned in, when you lift, the fuel shuts off.  Exhaust is normally too rich to ignite, but if you start pumping a small amount of fresh air, with no fuel into it (due to your idle set screw or IAC valve) it will periodically create an ignitable mixture, and that's the POP.

The only way to eliminate it is to dial in a little over-run, because richening up your idle mixture will just make it run stupid-rich when you're actually at idle RPM.

As for the 07, what's your exhaust system like?  They run crazy rich on startup, and the unburned fuel could be mixing with the fresh air still in the exhaust until it hits the magic ratio and lights off.
View Quote


Makes sense, it was MAP, I did no tuning to it, thing would do it intermittently anytime you closed the throttle and it was rolling, from coming up on stoplight, to rolling a mile or more down a hill.

07 is stock thru the single 3" resonator then a FM 40series to 90 out the side, just something cheap I put on so I could hear it without being too obnoxious, system length was reduced about 6' in the process. It doesn't do it much, only if you snap the throttle on off or go from hard accell, which probably makes it enrich on top of the already rich start-up, to abruptly completely off the throttle, in a low gear, it clears quickly, second, maybe less. No doubt it's the same issue, throttle closed above idle I'd think it'd go to almost zero fuel.
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