Straight pipe with 3rd catalytic converter delete. LOSS IN MPG

[Thuggins]

2015 GMC Sierra 1500 SLE   31K miles. So I straight piped my exhaust and deleted the third cat now I get about 9 mpg literally went through half a tank in one day. Can anyone shoot me in the right direction as to why this is! My guess back pressure but I keep reading things that go back and forth about it. Long story short I just bought a new cat and borla atak full system to try and fix this issue. Running 93O 

[KARNUT]

2015 GMC Sierra 1500 SLE   31K miles. So I straight piped my exhaust and deleted the third cat now I get about 9 mpg literally went through half a tank in one day. Can anyone shoot me in the right direction as to why this is! My guess back pressure but I keep reading things that go back and forth about it. Long story short I just bought a new cat and borla atak full system to try and fix this issue. Running 93O 

Did the same with my 14 no change in mileage. After the front cats tho. Went back with the muffler to quiet it back down. Been doing exhaust since the seventies, got better never worse, except if I was dogging it more.


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[Thuggins]

I haven’t been running it hard either. 

[Ozer]

Youre probably running rich, not sure how you get around the O2 sensors once you delete everything. 

[1SLOW1500]

3rd cat shouldn't effect anything main cats is another's story. They make a product called O2 simulators they plug in the harness and on the end is just a small black box. Zip tie up and away from heat. But in your case not the issue. There is a warm up where the rear sensors are looking for heat. I turn this off in tune as it runs rich as stated. I would reset the pcm so it can recalibrate the short term fuel tables.
Other thing is what material you used. Sounds silly but stainless is loud and doesn't warm up as fast.
When you say straight pipe. How long are you talking?

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[Thuggins]

2 hours ago, Ozer said:

Youre probably running rich, not sure how you get around the O2 sensors once you delete everything. 

No 02 on the third cat 

[Thuggins]

42 minutes ago, 1SLOW1500 said:

3rd cat shouldn't effect anything main cats is another's story. They make a product called O2 simulators they plug in the harness and on the end is just a small black box. Zip tie up and away from heat. But in your case not the issue. There is a warm up where the rear sensors are looking for heat. I turn this off in tune as it runs rich as stated. I would reset the pcm so it can recalibrate the short term fuel tables.
Other thing is what material you used. Sounds silly but stainless is loud and doesn't warm up as fast.
When you say straight pipe. How long are you talking?

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I had the flashpaq on but tried resetting to stock still the same effect. And about 4 foot of steel pipe 

[Jacoby]

I bet that truck sounds like ass. 

[VdellaV]

I would get a tune, regardless.

 

A tank of fuel should last around 8 hors of driving, what does your dash fuel MPG meter say?

 

Maybe you're using more fuel because you like the sound and are pushing the throttle more.

[1SLOW1500]

I am not talking about reset to stock. You need to reset the fuel trim tables. And at same time a tb% and trans. A real tuner would help just click the cat warm up box or delete rear O2 out all together.

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[Thuggins]

3 hours ago, Jacoby said:

I bet that truck sounds like ass. 

Wrong. It’s not even loud haha 

[VdellaV]

1 hour ago, Thuggins said:

Wrong. It’s not even loud haha 

 

This is correct, back when I had this exhaust set up, I ended up removing everything but the resonator, ended up sounding real good.

 

Since then, I installed Long Tubes and a custom magnaflow exhaust, ill leave the sound clips here, maybe they will be of help:

 

 

 

 

 

[Thuggins]

18 hours ago, VdellaV said:

I would get a tune, regardless.

 

A tank of fuel should last around 8 hors of driving, what does your dash fuel MPG meter say?

 

Maybe you're using more fuel because you like the sound and are pushing the throttle more.

It reads 16-17 average then drops down to 11 for a very short period then back up. 

[Thuggins]

14 hours ago, VdellaV said:

 

This is correct, back when I had this exhaust set up, I ended up removing everything but the resonator, ended up sounding real good.

 

Since then, I installed Long Tubes and a custom magnaflow exhaust, ill leave the sound clips here, maybe they will be of help:

 

 

 

 

 

Damn that sounds good 

[Jacoby]

20 hours ago, Thuggins said:

Wrong. It’s not even loud haha 

I didn’t say I bet it’s loud. I said I bet it sounds like ass

18 hours ago, VdellaV said:

 

This is correct, back when I had this exhaust set up, I ended up removing everything but the resonator, 

 

 

 

And yep you confirmed my suspicions