I went from 450 miles a tank before the fuel light would come on, to only about 390MPG before the light went on.

Since then I've changed the fuel filter, PCV valve, 7 of 8 plugs (I'm lazy I'll get to the last one soon), oil change, and Cleaned my air filter (volant)

After all of that, nothing changed much.

I also think I'm down on some power, based on my highly calibrated butt-dyno ? I do have a Westers Tune, and am planning on getting a re-flash with some updated programming to reflect some upcoming changes, but this mileage decrease has be befuddled.

Do any of you have any ideas?

This is your run of the mill 5.3L with about 80k miles on it.

 

Milage? 02 sensors?

I used to get high 300s in my Z71 but as time wen ton it got worse. My lil grey truck can only get about 370 before the light comes on.

 

i ran some injector cleaner in mine and picked a few miles back up.

 

I had the same thing happen. Tried new plugs, wires, o2s, and cleaned fuel system. None of the above helped.

 

If the air filter is a K&N or another oiled gauze type, you may have an oil film on your MAF sensor element.
I would do a few WOT runs to make sure the filter was free of any over oiling and then contact clean the elements.

Try it, you never know.

 

Double check your plug gap? Just a suggestion

 

Bingo, We have a winner. I did the same thing. Just clean the MAF.

 

I read on here somewhere that the gas we get during the winter months is not as "good", especially around large cities. Just a thought.

 

Wish mine got 390. It only gets 270 or so tops

 

If I'm good, I get 280miles.