Loosing more and more MPG!!

[Jolly Roger]

My MPG just seems to keep going down no matter what I try.

2007 Tahoe 5.3 Flex Fuel
4x4
AFM delete with tuner
73k miles, great condition, Government fleet maintained, no rust.
Code P0446 always on, nobody has been able to fix it...No other codes thrown ever, just evap.


I could have sworn I was getting 16 MPG about two years ago right before I developed a noise that sounded like it was coming from the transmission.
Had it checked out and they replaced the front differential.
Noise was still there so another shop replaced the transfer case.
Somewhere between these two repairs my gas mileage went from 16 down to 12.
One of the shops who looked at it said they tangled an O2 sensor wire in the drive shaft but fixed it.

Fast forward two years, now I have lifted the truck 2.5 inches and put 33 inch tires on it and a light bar on top. Im down to 11.5 MPG now

A few weeks ago I had the following done: $$$$$

Brake pads and rotors front and rear
Brake fluid changed
Spark plugs and wires
oil and filter change
Both belts changed
Transmission filter and fluid change
Front and rear differential fluid changed
Coolant change
Power steering fluid changed
Tires balanced and filled up a bit more from 48 to 55 psi (load range E)

--------------------

I told the shop that it was running rich and suspected a problem with the O2 sensor or wire (I told them about it possibly being damaged years ago)
The mechanic asked if it was FLEX fuel (yes) he said they have problems with locking in a fuel trim level and need it cleared/adjusted.
I also told them about the code that has been on sense the day I bought it (evap) I told them I wanted the lines blown out and checked for charcoal bits because all the components of the system have been changed 4 times over now including gas caps and even a gas filler nozzle. Typical symptoms of not being able to fill the tank without the pump kicking off several times...maybe,..just maybe if they blew the lines out I wouldn't have to stare at that check engine light anymore...

I picked the truck up after everything was done and he said that the fuel trim levels were indeed off, (14) and he set it to (3) I think..? supposedly nothing was wrong with the O2 system.
He said they blew out the evap line and nothing came out, reset the code...

NOW....

My peddle seems to be less responsive now, my CIL came back on within a few miles (evap same code) and after driving it for a few weeks I have just witnessed my MPG go down AGAIN from a 11.5 to a 11.2!!
I haven't noticed any performance gains whatsoever. (was hoping for a miracle with new wires and plugs..)

After all I just had done I thought Id at least gain a few MPGs!

Anything come to mind after hearing my long winded story?

 

[Jolly Roger]

I have a Superchips Flash paq tool. It allows me to see codes and clear them, change tire size for the speedometer and turn off AFM. I return the truck to stock configuration before I get it serviced...

 

[Foggy]

33's and a lift would drop your MPG prob 1 or 2.. The light bar, prob only at
hwy speeds maybe a hair...
You might want to clean the MAF and the Throttle Body...
I would suspect that your upstream Oxygen Sensors may be off, esp
if you've been running really rich... O2 sensors are kind of a tune up item..
I don't see the evap code causing any MPG issues, although very annoying..

I wonder if the shop that jacked up the O2 sensor wiring got into the vehicle
side of the O2 and "repaired it"... You can't extend or really repair O2 wiring as
it screws up the relationship of the voltage... Check to see if that's the case..
If so, find used wiring harness for the O2's or see if gm still makes it

 

[Luie]

Whats the weight difference in the current tire/wheel combo vs. what was on it? When I put some 20s with 305s on my Dmax in place of the factory 17s with 265 I lost fuel mileage. The 20 are heavier..

 

[Fless]

You need a scan tool that can read fuel trims. No way the tech "sets" or resets them; the PCM does that. There are many videos describing how fuel trims work, here's one:

He said it was at 14%; that's a bit high and indicates a possible vacuum leak or unmetered air getting past the MAF. But you have issues with the evap system and that can mess with the mpg, too.

If you're not running ethanol in a high content, and your PCM is mis-using the Flex table, it would run like crap.

 

[GreyStone]

Jolly ~ sometimes those can be the symptoms of plugged up cats

 

[BG1988]

My MPG just seems to keep going down no matter what I try.

2007 Tahoe 5.3 Flex Fuel
4x4
AFM delete with tuner
73k miles, great condition, Government fleet maintained, no rust.
Code P0446 always on, nobody has been able to fix it...No other codes thrown ever, just evap.


NOW....

My peddle seems to be less responsive now, my CIL came back on within a few miles (evap same code) and after driving it for a few weeks I have just witnessed my MPG go down AGAIN from a 11.5 to a 11.2!!
I haven't noticed any performance gains whatsoever. (was hoping for a miracle with new wires and plugs..)

After all I just had done I thought Id at least gain a few MPGs!

Anything come to mind after hearing my long winded story?

10-12 MPG city is normal for the 5.3L engine most areas have adjusted the traffic lights so you burn more
fuel

the worst i have ever gotten in my 3.8l was 12MPG in the metro area years ago so yeah your tahoe is sipping fuel.. even the 4.8l sips fuel more efficiently

It's a Conspiracy to boost up the oil/gas prices


Now on to the CEL
P0446 is a restriction in vent "solenoid" this is a electronic code not a cat code(if it was the cat you would have a P0420/P0430 code)


so you have an issue with one of the following:
Baro(MAP sensor)
MAF sensor
IAT sensor
ECT sensor
Ambient air?
TPS_FA (Throttle Position Sensor)
Vehicle speed sensor
engine power limited

 

[wjburken]

10-12 MPG city is normal for the 5.3L engine most areas have adjusted the traffic lights so you burn more
fuel

the worst i have ever gotten in my 3.8l was 12MPG in the metro area years ago so yeah your tahoe is sipping fuel.. even the 4.8l sips fuel more efficiently

It's a Conspiracy to boost up the oil/gas prices

 

[BG1988]

View attachment 252129

yes the traffic lights have been adjusted in pretty much most areas even in Los Angeles they have made an adjustment to the traffic lights(don't even have to press the walk button anymore)
it's been switched to "night mode" aka "GAS Guzzling mode"
citing LADOT
https://ladot.lacity.org/coronavirus/relaxed-parking-restrictions-extended-july-6


12MPG in the city is right on with the 5.3L


also here is the TSB on the code
http://downloads.innova.com/tsb-files/80000/2380966.pdf

 

[wjburken]

yes the traffic lights have been adjusted in pretty much most areas even in Los Angeles they have made an adjustment to the traffic lights(don't even have to press the walk button anymore)
it's been switched to "night mode" aka "GAS Guzzling mode"
citing LADOT
https://ladot.lacity.org/coronavirus/relaxed-parking-restrictions-extended-july-6

I’m not arguing that the timing of lights may have been adjusted, but to say that the reason is so people burn more fuel to drive up gas prices is what I’m called BS on.

In a lot places the pedestrian buttons at traffic lights are pretty much disabled because all lights are synced up with the greater system.

https://www.google.com/amp/s/www.bl...pedestrian-push-to-walk-buttons-actually-work