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Fuel economy loss mystery...

9K views 7 replies 6 participants last post by  RTOB 
#1 ·
I have a 2008 GMC Sierra 3500HD 6.6 turbo diesel that I have recently started to mysteriously lose mpg with. It is getting up there in miles (476,000) and I do a lot of RV hauling with it. Over the last month and a half I have noticed a random loss of 3-4 mpg. Before this, I was always getting 12-14 loaded and 19-21 unloaded highway driving. Now I am getting 9-11 loaded and 15-17 unloaded! I have not noticed any power loss and I put an EDGE on the vehicle and did a forced regen and all my injectors balance rates were normal. I have also used injector cleaner but it did not change anything.

However, ever since I have owned the truck I have had weird problems with my MAF sensor. Whenever it is damp out or cold when I first start the truck up I usually get a MAF sensor circuit control error and the truck is in reduced engine mode. I usually have to wait 10-15 minutes and then clear the code and it doesn't come back. When this first started happening I brought it into the dealership and they told me the sensor was fine and that I had a bad wire, and that they fixed it. About a month later I started getting the same results when I first started the truck up and it was cold or damp out. It has never affected my fuel economy though and I have just lived with it.

So my question is, now that I am experiencing this mpg loss, could it be my MAF sensor has finally gone bad? I should mention that I did try cleaning it when I first got the truck and that did not seem to change anything. Could it be something else causing this? What do you all think?
 
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#2 ·
Is the truck tuned? Winter fuel can cause a loss of MPG. When was the last time you changed the fuel filter & or air filter?
 
#5 ·
A bad MAF sensor can definitely cause poor fuel mileage. Same with winter fuel - I don't get as good fuel mileage with it. I'd think if your MAF sensor isn't good and with winter fuel a 3 mpg loss is not bad! lol. But, if you've never really noticed it before I'd probably start by getting a new MAF.
 
#6 ·
Replace the MAF sensor with an OEM 1. & yes a loss MPG from winter fuel is noticeable.
 
#7 ·
Yea I have always lost about 3 mpg in the winter here in MO. That being said if you want to run something good through the fuel system grab some archoil off powerstroke help.com they market it for fords but it's an insanely good lube and cleaning system I picked up about 1 mpg highway after 2 tank treatments and that was in December.
 
#8 ·
I own a 2019 Denali 2500HD and I now understand that winter fuel will cause a loss of 3-4 mpg when used in summer but I cannot understand why my fuel economy will drop from 23.5 MPG (10l/100km) to 13.5MPG (17.5l/100km) for no reason. No change in load or driving habit. During winter it appeared after the first cold start of the morning but as the engine reached operating temp it settled back to normal. I drove 182 mile round trip last Wednesday and had 23.5MPG. Saturday was 73 F here and for the short trip of 50 miles, I averaged 16MPG. This morning was 52F and I immediately was getting 23MPG until I turned on the engine break about 10 miles in and it dropped to 16MPG or so and never came back even after turning the engine break off. I am confused....
 
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