DFM Drops while pulling trailer terrible gas mileage

SLR88 · August 24, 2020

About to loose my religion.....Just purchased used 2015 GMC Sierra Denali 5.7 With 255/75R20’s Pulled a lightweight enclosed trailer and got 8-9 mpg. I could actually see needle move...Engine constantly jumped to 4,000 rpm on very slight inclines. I’m assuming I’m having cylinders drop???? Even when trailer switch is engaged.. If not DFM would changing to 18” tires help???? Thanks

JimCost2014 · August 24, 2020

18 minutes ago, SLR88 said:

About to loose my religion.....Just purchased used 2015 GMC Sierra Denali 5.7 With 255/75R20’s Pulled a lightweight enclosed trailer and got 8-9 mpg. I could actually see needle move...Engine constantly jumped to 4,000 rpm on very slight inclines. I’m assuming I’m having cylinders drop???? Even when trailer switch is engaged.. If not DFM would changing to 18” tires help???? Thanks

Welcome to the site.

Just a few questions, do you have a 5.3, or 6.2, no 5.7 in the K2's, are you referring to AFM, DFM is for the T1 platform, and 18 wheel/tire combo will be the same circumference as the 20's you have now, so that will male no difference.

Define what you mean by cylinder drop? Any load on the engine, and AFM will not let it go into V4 mode.

CamGTP · August 24, 2020

Let's assume he meant 275/55/20 tires as well because 255/75/20's aren't a thing as far as I know.

Tow haul mode will probably keep it out of going into 4 cylinder mode too. And tow haul mode will always try to put you in the best power band when pulling a hill. So having it downshift more often isn't that abnormal.

Also it may be a light weight trailer but if it's still a giant brick, it's going to reduce your mileage a lot. And if you go 70-80mph, good luck getting any decent fuel mileage while towing. Back it way down to 60-65mph and watch the MPG increase.

SLR88 · August 24, 2020

I appreciate your replies. It’s a 5.7 and I am referring to the AFM. Thanks for the correction. Also referring to down sizing to 255/55R18 I calculated tire size to be about 2” smaller. Not sure of the rear end gear size....Dropping cylinders I’m referring to AFM reducing cylinders. And this may be a non-issue other than a gas hog because like I said I just purchased this vehicle coming from a Chevy 6.6 3500HD dually and it is day and night difference. With that said I probably need to relearn how to drive a non diesel vehicle.

SLR88 · August 24, 2020

Also, by the way, when it kicks in “passing gear” and increases the rpm’s, there’s no increase in speed...it’s all acceleration and no torque. Like it’s stuck..this it why I thought I was having issues with the AFM. I’m just wanting to rule out electronic and mapping issues before I accept that it’s all in my head after many years with my 6.6 diesel.

LDM · August 24, 2020

11 hours ago, SLR88 said:

I appreciate your replies. It’s a 5.7 and I am referring to the AFM. Thanks for the correction. Also referring to down sizing to 255/55R18 I calculated tire size to be about 2” smaller. Not sure of the rear end gear size....Dropping cylinders I’m referring to AFM reducing cylinders. And this may be a non-issue other than a gas hog because like I said I just purchased this vehicle coming from a Chevy 6.6 3500HD dually and it is day and night difference. With that said I probably need to relearn how to drive a non diesel vehicle.

You do not have a 5.7L engine in your 2015 Sierra. The only options for the Denali trim in that year is the 5.3 and the 6.2 engines. They 5.7 hasn't been put in a vehicle since 03, when it was put in the last of the old body style Express vans. The last time it was in a Sierra would have been 1999.

Also, AFM would not be activating when towing something, it only activates under light loads. It would have nothing to do with effecting the gas mileage you get while towing a trailer.

CamGTP · August 24, 2020

Do not downsize your tires either. That will make everything worse.

The speedo will be all wrong. It will throw the transmission for a loop because the tire size is completely different than what it thinks.

Stick with a factory size tire.

evad · August 24, 2020

troll

newdude · August 24, 2020

18 hours ago, SLR88 said:

About to loose my religion.....Just purchased used 2015 GMC Sierra Denali 5.7 With 255/75R20’s Pulled a lightweight enclosed trailer and got 8-9 mpg. I could actually see needle move...Engine constantly jumped to 4,000 rpm on very slight inclines. I’m assuming I’m having cylinders drop???? Even when trailer switch is engaged.. If not DFM would changing to 18” tires help???? Thanks

So. AFM doesn't work above 3000rpm or so on your 5.3 V8. You can watch the V8/V4 indicator on the dash as well in the fuel economy screen.

Did you have Tow/Haul turned on? You should if you don't. It will keep the engine in the powerband better so it won't kick down on you. Do you live in a hilly area?

Changing wheel size won't help.

da90 · August 24, 2020

I don’t use the tow haul when empty or small trailers. It’s shifts too much on small inclines or braking. When loaded the I definitely use tow haul.

Sent from my iPhone using Tapatalk Pro

Colossus · August 24, 2020

20 hours ago, SLR88 said:

About to loose my religion....

SLR88 · August 25, 2020

Correct it’s a 5.3. And thanks for all the positive feedback..Like I mentioned it’s probably a non issue, I just need to understand I’m not pulling with my 6.6 anymore.

And E the troll yahoo, I’m not sure your ridicule is welcomed in this forum.

Edited August 25, 2020 by SLR88

JimCost2014 · August 25, 2020

1 hour ago, SLR88 said:

And E the troll yahoo, I’m not sure your ridicule is welcomed in this forum.

Don't let the normal welcome wagon get you down, ask questions and learn about your truck.

Lots of good advice on here if you "weed" through the typical forum rederick.

Edited August 25, 2020 by JimCost2014

evad · August 25, 2020

2 hours ago, SLR88 said:

Correct it’s a 5.3. And thanks for all the positive feedback..Like I mentioned it’s probably a non issue, I just need to understand I’m not pulling with my 6.6 anymore.

And E the troll yahoo, I’m not sure your ridicule is welcomed in this forum.

ok