mileage problem

[Burner]

I have not been getting good fuel milage, at all. I have read that many of us are not getting the MPG that we had hoped for. :( I am wondering if the % thing (oil life) on the dash has anything to do with it? I have rest mine at every oil change, except for this last one. My mileage seems to be increasing? The truck is around 49% oil life, the lowest it has ever been. Is it possible that the computer is reading this? I don't know if it was capibable of doing that? Could it be part of a long term program? ------- How does the "dump" or return work? Can it be clogged? Would not most of the return be at lower RPM's? If the dump were limmited or pluged, what happens to the fuel? I just do not understand how these truck can vary soooo much in fuel mileage? :confused:


Burner

[bullet2110]

I'm with you burner...mines going to the dealer tommorow....will let you know if I find out anything that may help.

[K1XV]

Sorry to hear you fellows are not doing as well as some others, but remember that those most likely to post high mileages are braggers (like me?) who are doing well, and those who are at the other end of the curve and doing not so well. On the other hand, I have some other issues, such as the "Service 4WD" message, and I am getting a new encoder motor, which may or may not solve the problem. So we all have our crosses to bear.

In any event, I think aerodynamics may play a part. I have a stock crew cab, and when I put an A.R.E. "Z" model cap on it, I seem to have picked up a mpg or so. Also, I see mileage suffer with sustained speeds over 75. Also, going up in tire size from 245 fools your computer into believing you have covered fewer miles than you have (unless you got it corrected), causing the mpg read out to register low, and may cause real mpg to suffer as, all other things being equal, as wider tires push more air and create more drag. The toolbox adds weight, and may add aerodynamic drag. I am doing between 18 and 21.5 mpg on sustained trips. This is with the stock Firestones on the factory aluminum wheels, and the cap in place. If I am driving around in the hills of Southern Vermont on a cool day, 16.5 is more typical.

Oh, I forgot. The length of your average trip also has an impact. Our Duramax engines hold over 20 quarts of water, and 10 quarts of oil. Aside from the engine itself, that is a lot of fluid to warm up.

My cars show normal water operating temperature within 3 miles of start up. With the Duramax, by the time things are getting into a normal temperature range (which can take, in my experience, up to 10 miles or more from a dead cold morning start up) I am only getting about 14 mpg during this initial 10 miles. :rolleyes:

I suspect if your typical drive was interstate at 65 mph you would do better.

Ray

[NWDmax]

I agree with K1XV.I live about 10 minutes/5 miles from my business and its all in town miles.I start the engine and run the high idle at the 850 rpm setting for 10 minutes before I leave.The temp gauge reads 160 and doesn't fully warm up till I'm at work.Its probably the worse case scenario as far as mileage is concerned and I average 12.25 mpg.Highway only mileage running empty is 17.25 to 17.50 @ 70 to 75 mph.Loaded with my Lance 820 camper(truck and camper scale @ 9800 lbs)and my 18' bass boat(approx 3000 lbs)and trailer I get 10.5 to 11 mpg.I do run the FPPF cetane boost and total power in every tank.I mix the two together and put 8 oz in the tank when I fill up.I always use the same pump at the same station and have the truck parked in the same position every time.I feel this is the best way to get the most consistent readings. smile.gif

[Burner]

K1XV, I to have had the "switch" problem. Mine seemed to fail comming home from an AK hunting trip. I had driven about 150 miles with no problem and then the message came accross the board. I knew that there was something awrey because the cruise controle was on 70 at the time. Both my 2 wheel and 4 wheel high lights were on. After 3 different stops and many forward, backward and key off/on it stopped. Turning the truck off for 5 minutes and starting the truck "without" placing it in gear for 5 minutes seed to do the trick. Al fluid levals were full. :rolleyes: On this 320 miles trip I avg only 13.8 MPG. I was pulling a small single axel trailer of 4,000 lbs total weight. I think that I had plenty of time to warm up the motor. What is an encoder motor? :confused:


On the airflow and weight part. Something just isn't right. It's just hard for me to beleive that 1,000 lbs in the bed could make that big of a difference in Hwy mileage. Seems to me that the truck would be more leval, reducing the power loss at the yoke. The wind will have some affect but not that much. If a 6.0 stroke that has more frontal area and more weight can get 18mpg then I think that our 6.6 should be able to get the same.


Burner

I have yet to take my truck in for service. I will, when I have time. :(

[K1XV]

Burner, as I understand it, the encoder motor is the actual device on the transfer case that shifts it between 2 hi, 4 hi, and 4 low. Funny, I have the shop manuals here, but have not had time to look at them.

Yes, my problem comes and goes with the lights deciding to change. It has happened 3 or 4 times in the last month. Sometimes it is the neutral LED that is lit up. If I try to go to another position, it just blinks.

The service advisor told me to not use 4WD "unless I really have to" until the encoder motor is changed. The funny thing is, I am not sure if I could get into 4wd if I had to. Thankfully, with the locker rear, have not had to go to 4WD to get around. But I don't like the idea of a 2wd 4X4.

Periodically, I get the "Service 4WD" message on the message display, which I simply clear.

It would not surprise me to see a recall when they figure out a permanent fix, either software, hardware, or both. And this goes beyond the Silverado and Sierra. I have read about someone with an Avalanche that has had the problem.

It is pretty chilly this winter in Vermont, with a colder than average winter. How has Alaska been?

[chuntag95]

An encoder moter is a motor that "knows" how much it has moved. They normally have a wheel with little slits and an optical sensor that counts them or a magnetic setup to count the "steps". That way they can drive to a certain step count and repeatedly hit the same spot. I use them on equipment at work to precisely move material in and out of equipment.

End of lesson. :D

[Burner]

K1XV,


I goofed and you cought it. I had been in Arkansas not Alaska. tongue.gif


The trip was really cold but fun.


Burner

[SledZep]

I drive 40 miles each way(35highway & 5 local) to one job (5 days) and 65 miles (60 highway & 5 local) each way to the other (weekend)
I cannot really get above 15 or 16mpg??

On the other hand slowing down to where I cannot stand it usually means 77mph or so...

I stopped checking my mileage because it was so dismal. I have since slowed down from 85-88mph to 75 or so consistently but have not checked my mileage. I have around 25K on the truck with maybe 1,000 miles of light towing.

On a fun note, I work off hours so I do not get alot of traffic and until my ticket (recent)I regularly hit the limiter either on the way to or from work.
Couldnt really complain till fuel hit $1.99 :eek:

[SPICER]

Burner and SledZep,
Check out this link, www.thedieselpage.com/mileage.htm. It is very educational about the variables contributing to fuel economy. Among other things it states that speed is a significant cause of decreased mileage, more so than on a gas engine. In the 50-75mph range we willlose 3mpg per 10 mph. this goes up at speeds over 75. If you are going 85, you will probably see a 9mpg drop from 55mph. Gearing has a lot to do with it also. I dont know your rear axle ratio, there are a few options, but differentials geared for heavy pulling will suffer on the highway.
I agree that short trips hurt fuel economy. SPICER

[Diesel Girl]

This may totally not apply but properly inflating the tires on my truck has helped increase mileage substantially. Even though I always ask that the pressure be checked when I take it in for a tire rotation, they never do it right. Finally got done right here last weekend and my SO said it got incredible mileage towing our 3 horse gooseneck trailer with 4 motorcycles in it to Daytona Beach.

Just a thought.

[K1XV]

SledZep:

15 or 16 mpg at 86-88 mph is not poor mileage in my book!

Ray

[RealBigTruck]

K1XV,

I agree with you! The last 3 tanks on long haul highway drives averaging from 75-80mph I have hit 16.8mpg each time. I'm pretty happy with that.

[Burner]

Most of my driving is "in-town" freeway. My speed will vary from 50 to 75 MPH. I really do not speed.... er.... that much :D :D

Burner

[c5dura]

Dropped the truck off at the stealer this morning and mentioned that I was only getting 11.5 - 12 mpg.

Service writer said that sounded pretty normal. I then mentioned that others seem to be getting 15-16 when running unloaded, which was backed up by my sales guy, so I'd like a second opinion.

So he calls over "Steve", one of the Duramax techs. "Steve" also tells me that 12 is pretty good for a big dually like mine and then he looks at my tires (BFG 235-85-16 ATs) and says that isn't helping any. I explain that worst case those reduce mileage by perhaps 4% or so, no where near enough to explain why I'm 20% or more down on mileage from the 'average'.

Then he asks what route I drive, and I mentioned that I take the Interstate across the Blue Ridge Mountains to work each day and he goes on to say that is the problem right there.

I mentioned that others drive a similar route across a 6%+ grade every day and are still seeing 15-16 mpg. At this point he becomes irritated and just states he'll check into it and let me know.

Next time I'm going on a road trip that does not involve mountains, I'll be sure to note my mileage and then go back and talk to "Steve" and/or the service manager if I still can't break 12 mpg...

Oh, and I handed them my fog light with its broken bracket, and they said that is not covered under warranty, but they would see what they could do...

The only positive thing that happended, and the reason I brought in the truck in the first place, is that they agreed to replace the steering/gear box under warranty since the top cover seal is leaking.