Please help, unfixable fuel mileage

Skylardog123 · June 13, 2019

Hey guys. So im suffering from horrible fuel mileage with my truck, its a 99 silverado 5.3 4x4 with 3.73s and 33 inch tires, when i bought it, it got a solid 10mpg, now im getting around 9- 9.5, and the inside of the motor is perfect. No sludge or carbon buildup or anything, runs and shifts great no noises or anything.

So far i have done everything i can to change it

02,sensors fuel filter, oil filter, air filter, knock sensors, cleaned fuel injectors, new plugs, brand new cats, seafoamed it, fresh and full oil all the way around, i drive it very easy and never stomp on the gas, i even have an hptuners custom tune on it and even that didnt change gas mileage, im stuck and not sure what else to do, im at my witts end with this truck and i would love to get at the most 15mpg. Doubt i can but im shooting for more than 9.5, any help is appreciated, thank you

sefiroxx · June 14, 2019

Check for vacuum leaks, clean throttle body and butterfly, clean the MAF.. Run a scanner while driving and monitor O2 voltages and fuel trims to see when the rich condition occurs.

16LT4 · June 14, 2019

1) how many miles have you put on since the purchase?

2) have the tires/wheels been installed since you purchased?

3) have you ever corrected your speedometer/odometer for the larger tires? If not, your odometer is under-reporting miles driven and therefore under-reporting mileage.

4) how long have you noticed the decrease in mileage? coincide with changing temperatures/fuel blends? 0.5-1 mpg is not much to be concerned with. Your truck, as equipped, getting that mileage doesn't sound too far off... the best I'd muster with mine, stock with under 50k miles when I had it, was 18 mpg on pure highway (like over a full tank with the cruise set to 69 and no stops). I could easily see 33s knocking that down considerably.

5) how many miles on it?

6) how are you *sure* the motor is "perfect" inside... did you pull the valve covers? send a boroscope down the spark plug holes? compression test it?

7) ever change the drivetrain fluids? on my 60k mile '14 Ram Hemi CC 4x4, I picked up 2 mpg when I replaced the differential fluids with Redline full synthetic. Also check for dragging brakes (especially the parking brake).

Skylardog123 · June 14, 2019

212k miles, pulled valvcover and yes i used a borescooe camera and looks amazing for the miles, i habe replaced all fluids, speedo is corrected and i put wheels and tires on it myself, no vacuum leaks, clean perfectly functioning maf sensor, cleaned TB 100% about 3 days ago

davester · June 14, 2019

also check for exhaust leaks ahead of the O2 sensors. The exhaust can suck in clean air, and it fools the O2 sensors into thinking the engine is running lean, so the ECM increases how much fuel is put in. Broken off bolts in the the exhaust manifolds is a common cause of this, but it can also be the exhaust manifold to exhaust pipe gasket as well.

Skylardog123 · June 15, 2019

I do have 1 stud broken on each manifold by the fire wall but i cant imagine it bringing me to a horrible 9.5 mpg instead of the average 15

riverbanks · June 15, 2019

Don't rule it out,just saying,and they do sell repair kits,now to the real ?,how old are the front 02's?

davester · June 15, 2019

It could, or at least contribute significantly to it, as it doesn't take a lot of clean air to really throw off the reading, and then the ecm will just start dumping in fuel because it's reading that the engine is too lean. And if the air leak isn't big enough to cause the ecm to hit the fuel trim limits, it won't generate a code, you just wind up running the engine really rich all the time.

Skylardog123 · June 16, 2019

Im not sure if they sell kits, i plan on removing the heads to drill them out one dat, and all 4 o2s are like 3 months old

MaverickZ71 · June 19, 2019

Wheels and weight/rolling resistance of the tires by itself could easily account for 0.5 to 1 mpg difference. New tires of any type, especially LT rated mud-terrain tires, have increased rolling resistance as compared to an all-terrain or highway tire or old, worn-out tire. Also, around here, the difference between Winter gas and Summer gas (EPA smog crap for metro areas) and any associated ethanol content can also account for a mpg difference.

Obviously, to get a 50% increase (up to 15mpg) in fuel mileage on a truck that is well-maintained and 'running right', you're looking at a change in ride height/tire size-tread-weight/transmission/gearing/aero drag/driving style/truck owned/all of the above.