Failure Without Codes?

[ThumperTDC]

Got a question about my poor gas milage. I have a 99 GMC 2500 with the 6.0L with 105k on the clock. I have recently installed a AFE stage 2 intake, a magnaflow dual 3 inch exhaust and 265/70 R17 tires. I know the 6.0L is not know for good milage, but prior to installing the exhaust I was getting around 14 city and 15 highway. After installing the exhaust the best I have gotten is 12. I would have thought by opening up the exhust I would have seen a slight gain in mileage, but not a decrease. Can O2 sensors be off without throwing any codes? I have also ran a couple of tanks through with injector cleaner hoping that this might help some. I have installed new plugs and wires, but have yet to install a new fuel filter. I am simply looking to get my mileage back to where it was if not a little more.

 

Any thoguhts on what I could check. I am planning on switching over to snthetic engine oil at my next change, and probably should get the tranny serviced and my switch over to Amsoil. should I look at my injectors? This is my first high mileage fuel injected rig so I am just trying to look at everything. I am looking into a programmer or custom tune for it in the near future, but what might be some things I could look at now to try to get the numbers back up.

 

Thanks.

[redvett]

These engines need back pressure to operate correctly, the 3" exhaust is your problem. Also if you didnt correct the speedo input for the larger tires your engine & transmission calibrations are off too.

[pennzoil29]

These engines need back pressure to operate correctly, the 3" exhaust is your problem. Also if you didnt correct the speedo input for the larger tires your engine & transmission calibrations are off too.

 

+1

[ThumperTDC]

My speedo is about half a tenth off, so its really not as far off as I initially thought it was.

 

As for the exhaust being too big and not having enough back pressure, I am not gonna have new pipe bend for it. I was thinking what if infront of the muffler Ihad acouple smaller pipes expanded on the ends to fit in line with what I currently have, but would restrict the exhaust a little giving it a little more back pressure......................does that seem like a good route to go?

 

I like the new power I have gained, this will help out when i start pulling the horse trailer for my wife.

[redvett]

More power = less fuel economy, and less back pressure raises the torque band, you need to keep the engine in the lower of the peak torque band to get best fuel economy.