2001 GMC Sierra 5.3 Poor MPG

[Vortec53]

I have a 2001 GMC Sierra with the 5.3 Vortec with about 211K miles. Over the past 5 months or so I've noticed the MPGs have TANKED. Be aware this is a Regular Cab 2WD! Early spring last year I was getting about 15-16 MPG with 75% hilly-city driving. Summer came around, I was still receiving 15-16 MPG. As of today, on my last fill-up I averaged 13.4 MPG. Previous two fill-ups were in the ~14.5 range. I very rarely pass the 2000RPM rev-range. I have a code that pops up every once in a while for a Downstream O2 sensor. I have a pending code for a UPstream O2 sensor. I've heard the Upstream sensors have a large play on MPG? Trucks rear end fluid, engine oil, and transmission fluid were changed May 2018. I also cleaned my Air Filter, MAF and Throttle Body about two days ago to attempt a quick fix.

 

Any tips will help.

 

Things I've also tried:

Seafoam (2 weeks ago) runs wayyy better

Marvel's mystery oil (mid-summer) did not change at thing...

 

 

 

[sefiroxx]

Computer uses upstream O2 sensor to make minor adjustments to the fuel trim.

You need to run a scanner while driving to confirm o2 and other readings as well as fuel trim

[dtnel78]

After you warm it up what's your high and low fuel trims? A decent low end scanner can give you this data. This will let you know if you're running lean or rich.

I'd scan it for codes. IF it's one of the ones with a fuel filter and it hasn't been changed in awhile I'd look there and also think about the air filter to begin with.

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