My 2004.5 LLY is stock, and now has a little over 11k on the odometer. From 8k to 10k miles I typically got 21 - 22 mpg per tank. I drive almost exclusively on the highway, always unloaded.

The last 4 -5 tankfuls have resulted in 18 - 19 mpg. Wondering if the colder weather plays a part (I live near Kansas City), I added a Lund Cold Front cover to get the engine operating temps up. I also ran Power Kleen additive every other tank to see if it helps. No change with either.

Should I worry, or perhaps do something else? I change the oil every 5k, but have not yet changed the fuel filter.

Thanks in advance.

Bud

Winter blended fuel will decrease your mpg. less BTU's in #1 diesel.

also, are you running any after market mods ?

My 2004.5 LLY is stock
Nope. No mods.

You may want to consider changing your fuel filter.

GM states fuel filter replacement @ 15K miles. Filter at my local auto parts dealer $33. DieselTech.com $22. AAhhh...the power of the internet.
Also...trans pre-filter $9. Stealership $22

Also, keep in ming that a grill cover is more windage. All that air that use to pass through the heat exchangers and out the bottom of the engine compartment is now pressure on the grill plate. Never discount the effects of added windage. It can add up quick.

For those of us in Kalifornia, the refineries have started lower the sulfer content of the diesel more and more over the last 2-3 months. My friend who owns the local diesel shop went to some GM duramax training and they made everyone aware of that as they felt it would start to decrease mileage...

great-:t

How dramatic a drop in mileage do you suspect it could be? Been having some really low counts lately, and am really unsure why, on avg, DIC calculates 12-13mpg city driving, and before, I was getting 14-16.. I hope this is just the case, Only started about a month or two ago for me.

Also, keep in ming that a grill cover is more windage. All that air that use to pass through the heat exchangers and out the bottom of the engine compartment is now pressure on the grill plate. Never discount the effects of added windage. It can add up quick.
Quite the opposite is true, Fly-Rod. There is less "windage" with the grille cover and the air is flow is smoothed around the grille and over the hood reducing drag. I've seen as much as 1 mpg improvement running the interstate with my grill cover on.

Sounds like all the "wrong" combinations are affecting our mpg's: low temps; lower sulfur; etc. Engines run more efficiently at higher operating temps. I live in Florida so extreme cold isn't a factor. I have noticed that on 40*-50* mornings the LLY can idle for 15-20 minutes and never get above 160-180*. Only way to get up to operating temp (205*) is to drive it. My mpg's suffer mostly because of my foot, just can't seem to let off that dang accerlator! JJ

I'm going to take a stab at changing out the fuel filter, and will see what happens either way. I suspect that I am suffering from low temps and low sulfur winter blend fuel, but will report back if anything changes.

I think most LLY trucks lose mileage as time goes on. Some of the better ones stay the same as off the lot. thanx geo.

LOSE? Crap, at 12.5 since new I better not lose mileage...

I started off averaging 16 - 17 per tank until around 6000 miles, when it went to 19 - 20 at 7500 and then 20 - 22 by 9500. This latest decline has me puzzled, as I "drive like an old grandpa" as my wife puts it.

There are a lot a variables with mileage. Traffic, wind direction, tires, alignment, weight, fuel, speed, etc, etc, etc.

But my guess would be winter blend fuel.

My first tank out of the dealer was up about 20 mpg, while after that it took several thousand miles to get it back up there. I've been running close to 20 mpg for about the past three or four thousand miles.

First tank 16, 8000mi later 15mpg. Driving mostly hy, no towing, no load. Expected better but then I didn't expect a fuel knock that 3 dealers can't make go away. If I were getting 20mpg I could probably live with the knock. Completly stock.

I think that the engine management problems on the LLY causes the injectors to clog (poor spray pattern) unless you get a long highway trip every day. That is my experience anyway. Thanx Geo.