AFM/Coasting Question

[Brandon Hoppmann]

I have a 2015 Silverado 1500 5.3L 6 speed. Recently I have noticed a couple of things- when I am cruising on the highway my truck will go in v4 mode under light gas like normal but when I take my foot off the gas and let it coast it actually goes back into v8 mode and won't drop back into v4 until I step on the gas again. It also feels like there is some kind of resistance when coasting, my truck seems to start slowing down significantly faster than it used to when I don't have my foot on the gas. MPG has also dropped from about 16 avg to 13 avg, I started noticing this about a month ago. Wondering if anyone has had a similar issue before I take it to the dealer and they tell me nothing is wrong because there have been no codes or lights.

[J-man67]

I'd would just go the safe route and have it looked at. Never heard of this, but you drive the truck and are the only one who would notice a change. If you feel something changed, more than likely something is not correct.

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[CamGTP]

There is a function within the computer called DFCO or Decel fuel cut off. When this happens it will shut off all the injectors and just turn the engine into an air pump without the explosions in the cylinder. It's pretty much an engine brake and will slow the truck down. Going into DFCO should disabled AFM/DoD, so it could be why it shows it going back into V8 mode.

 

Tough for me to comment on the mpg drop though. Mine is lower now because temps have been in the 20's every morning, winter blends for fuel are making it to the pumps and I use my remove start more.

[Grumpy Bear]

3 hours ago, CamGTP said:

There is a function within the computer called DFCO or Decel fuel cut off. When this happens it will shut off all the injectors and just turn the engine into an air pump without the explosions in the cylinder. It's pretty much an engine brake and will slow the truck down. Going into DFCO should disabled AFM/DoD, so it could be why it shows it going back into V8 mode.

 

Tough for me to comment on the mpg drop though. Mine is lower now because temps have been in the 20's every morning, winter blends for fuel are making it to the pumps and I use my remove start more.

BINGO!

[Grumpy Bear]

98.5% of the extra fuel use goes into heating the oil from much colder temperatures this time of year. 1.5% comes from the lower BTU content of winter fuel. FYI. 

[g2full]

Good to know the reasoning behind it. I noticed it with mine too, besides the mileage drop. Thought maybe mine was just busted.

[Ozer]

Ill have to check mine but pretty sure mine stays in V4 when i let off and going downhill, it shows 99mpg as im coasting. If you put the truck into Neutral then it will go into V8 mode for sure.

 

As far as drag, check your emergency brake, make sure your rear calipers arent stuck. My ebrake was stuck and i could feel a slight resistance before i did all brakes.