Huge mpg drop when towing on 2017 Silverado 6l80 5.3

Snowshuffle523 · December 10, 2024

I can’t seem to figure this one out. I’m getting around 5mpg when towing a 7x14 enclosed trailer with a UTV in it, roughly 5k pounds total. I used to be able to get around 8-10 towing at 70mph but it’s taken a steep drop off lately. My friend 2018 Silverado gets 8-12 when towing my trailer. Tire psi at 38 on the truck and 50 on the trailer (max rating for the tires). Engine has 88k miles, air filter is clean, MAF sensor in clean, plugs and wires were replaced about 3k miles ago. Combined my is roughly 15-16 when I’m not towing, all driving is done on mostly flat roads with only slight inclines. Truck is bone stock with no lift, running LT tires, does have a bed cap.

The data below was towing with no significant head or tail wind.

I did some digging with an OBD2, no active codes, short term fuel trims are around -1 to -5 and long term at -7 when cruising at 70mp with the trailer. Fuel pressure sits between 40-56 psi, engine load at 70mph sits between 80-100%.

the only thing I’m not sure of is the injector balance test, one side of the engine sits at 37 psi drop and the other side is 46 psi drop. Is that expected to have one side of the engines with such a spike in psi drop? I did some searching and couldn’t find an answer, I thought psi drop should be roughly +/-10% across both banks on injectors. Picture for reference

Edited December 10, 2024 by Snowshuffle523

CamGTP · December 11, 2024

I wouldn't really trust that balance data too much. Have to remember the low side pressure will be in that 40-70psi until it reaches the high pressure pump that turns it up to 2,000-3,000psi to fire the injectors.

All the other data seems fine, fuel trims seem good and all that.

5mpg is about the worst I've heard so far. I towed a car on a open deck car trailer, totally over 5,500lbs and I got at least 11-12mpg doing that. I would expect similar mileage with an enclosed trailer of your size.

Are you see abnormally high transmission temps while towing this weight?

Snowshuffle523 · December 16, 2024

Not seeing anything out of the ordinary but I need to pull it again soon. I’ll try to get some live data when towing over my 4 hour trip. I figure it can’t be too high or I’d expect it to throw a code, but who knows

I ended up taking it to my mechanic last week and they said they couldn’t find any silver bullet but the intake had some heavy oil buildup (I don’t have a catch can) and carbon, they did a clean on it so I need to do some more towing and see if that helps. I can’t imagine it will make a massive difference. I don’t expect a miracle for MPG but I’d love to get 8-9mpg when towing vs 5. If you have any other things to check I’m all ears! I appreciate your response!

Edited December 16, 2024 by Snowshuffle523

mikeyk101 · December 16, 2024

When did you notice the drop in mpg? Not sure where you are located but in the colder climates, the winter blend gasoline is what you end up buying now and there is definitely a drop in fuel mileage when that happens. Running at 70mph while towing will also affect the gas mileage so maybe try dropping that down a bit. Also, when I tow either my toy hauler or enclosed 7x16 cargo, I always bump up the air pressure in the truck tires which can help with fuel mileage as well. I run Falken Wildpeak tires 275/60R20 with a max pressure of 51psi and run with 44psi when towing. One final thing, many trailer tires are not really rated for 70mph and you could be rolling the dice with that. What trailer tires are you using?

Edited December 16, 2024 by mikeyk101