Mileage dropped like flipping a switch in early October by over 2 mpg.Have put on over 6,000 miles since and it is getting worse.Have kept mileage records since new and even for the winter months it was bad.Now it is warm out and the mileage is down by about 3 mpg.Took to dealer and paid to have injector balance rate tested,all between 1.4 and minus 1.6 also tested engine temp. sensor which was fine.Also tested engine idle with one cylinder shut down at a time with a 20 rpm drop for each cylinder.Thermostats seem to working properly with no fluctuation in temps.I have checked all the obvious things like tire pressure,filters with no luck.Engine starts and runs good with no smoke.I am stumped.Love the truck but am really getting frustrated here.Do not know if another is in my future(2006)if simple diagnostics can not solve the problem with this one.:help:

mine drops about 2 mpg when burning winter diesel ... the time frame you mentioned could be when the stations in your area started selling winter diesel


Jim

Previous two winters netted excellant fuel mileage on this vehicle.One January trip of over 1,000 miles netted a 20.9 mpg on three tanks,temps from 17 to 38 F.A 50 mile commute consistently putting out over 19.2mpg.I do not use no.1 mixed with no.2.only no.2 clear with addatives.Last summers mileage ran between 20.4 and 21.6 using a full tank weekly.

Air filter/ intake obstruction?

You might check your fuel filter. My truck shows a definite drop if it goes too long.

My '02 with over 108,000 miles on it consistently gets over 21 mpg on the highway. We don't get a winter blend of fuel down here cause we don't get that much of a winter I guess.

New airfilter,no restrictions.New fuel filter,changed every 10,000miles.

Have you had the fuel pump checked?

No, Im having the same problem. Except mine is a little more extreme, I went from 21-22, to less than 13. I checked my oil and I have 13 qts, and its really watery. You probably have bad injectors, My injectors balance perfectly sometimes and at others show as much as 8 over. There really wierd, takin it on monday for 8 new ones, I hope

Saywhat,
My mileage dropped 1-2 mpg in mid November and didn't come back up until the first part of April. The mileage follows the winter fuel blend mixture as supplied at the pump. You don't have any control of the blend on #1 & #2. The temperature and distance driven play a big part in the mileage. You should have noticed that when the engine/transmission is cold the torque convertor doesn't lock up in any gear until it warms up. Doubt that you have a real problem, just circumstances against you. I am down here in Madison so we should have about the same fuel blends.

SAYWHAT? So what is your milage down to? You say you were getting 20.4 and 21.6!! That's better than 99% of the Duramaxs on the road. If your only down to 18mpg you should still be happier than a pig in sh*t. I've never gotten above 19mpg in the 65K miles of my '02. Average 18 on long trips.

I have the same problem, have yet to figure out whats wrong. I'm taking it in to the dealer Friday to try and make sense of it. It started dropping since last October and has not snapped out of it. The best I have gotten in 5 months is 14 mpg, most of the time its 12.5-13. When I first bought it last July I was getting 17-19.

Same here. 12 towing 14 around town. 18-21 mpg... wow.

Last tank down in sixteens,filled up again today and was at 20.4mpg.Same 200 mile trip as previouis,same temps but raining.Always fill tank to the top of filler tube and hand calculate mileage.Miracle if it was the fuel,use two different suppliers with usually same results.My driving is almost all long hauls with short idling and consistent driving habits.About three weeks ago I swiss cheesed the air box to see if that would help,think it made matters worse.Started thinking this could be a temperture sensative matter so really kept an eye on the temp gauge for 74 miles this mourning,ambient temp at 50F and temp gauge would spike where it usually would, stay there a while then drop about 12 degrees or so wich it did not do before about last fall.Put new thermostats in this after noon and run her 20 miles,no more temp swings.Why the mileage jumped back on last tank,can not say for sure.Will have 200 miles on by weekend and will post results good or bad.

Forgot to add that I taped the swisscheese holes shut after that 16mpg tank of fuel.

Most places switch to summer fuel in mid to late April, even up here.

Well, took mine to the dealer and they say their is nothing wrong with it. They did do a update in the computer but they were not sure if it would do anything. I just don't understand it, I drive it very respectable too.

Since last post I put on 355 mlies and used 17.9 gallons for a 19.8 mpg result.Mixed driving,74 miles on interstate running 72 mph,241 miles on state roads running 55 to 60 mph,remainder in town.Seems back to normal as the weather has not warmed up around here yet.Forgot to mention that the dealership did upgrade engine computer prior to that 16 mpg tank.Tech said only info on upgrade was to address fuel economy and emissions,can not say if it did either.

This is my second diesel, and I had a similar problem with my first diesel. One lesson I learned with the Ford PS was that you have to really check your mileage different from a gasser to diesel.

Diesel foams more than beer, so you have to pack the tank until you see diesel standing in the tank neck. With my ford, the "pack" took almost 4 gallons of fuel, not packing the tank would create wild swings in the mileage figures. The dmax takes about 2 1/2 gallons to pack.

I've also noticed the greener the fuel the better the mileage. This clear, ultra low sulphur ain't worth a sh!t, Last month I got some good old regular green diesel in Ark and got over 20 mpg, which was a first for my truck.

My mileage usually goes up in the summer and down about 2 mpg in the winter. Plus these trucks really like clean filters!
Doug

Packing the tank gets better mileage? I never thought it was good to fill a tank that full. But it is ok to do that? I may try it.

Packing the tank on a gasser is a no no.It will saturate the charcoal cannister.As I stated earlier I fill my diesel tank to within sight in the filler tube.It takes a while to get rid of the foam but will give you an accurate mpg comparison from tank to tank.