D-Max Man, More Power, anyone else...

[7mmSTW]

We desperately need some help. As I have posted before we are having a problem. Our mileage started out great. After the first oil change we suffered a severe drop in mileage. We have always thought that this was a coincidence but it happened again after the second. The last full tank before our second oil change was 16.9 in town. We had 300 miles with more than a half of a tank left when it went in for an oil change at the dealership. After the oil change we made it 92 miles on what was left. This is a longbed pickup so it has the "big" tank. WTH could be causing this. Otherwise we love the rig but this will absolutely not do. PLEASE RESPOND.

Thanks

Seven :mad: :confused:

[srubrn]

First of all 16.9 in the city seems pretty good. The fuel always seem to last longer on the first half. I think it is due to bad sending units. GM products have always been this way as far as I can remember. What is the capacity of the "big" tank?

[Amianthus]

Maybe it's that eternal "first half of the tank goes away slower than the last half" thing. :D tongue.gif

Seriously, how many miles do you normally get out of a tank? If you are losing that many miles, you may want to bring this up to your dealer. That being said, I cannot think of any condition that can be brought about by a simple oil change that will result in the mileage condition you describe. Perhaps you have some sort of air filter restriction from dirt roads? Exhaust restriction? Do you let it idle alot? Do you get lots of smoke on accelleration? Do you use a fuel additive? Do you use a fuel station that moves alot of fuel (algae problem maybe?)?

These are some of the things that could cause that condition, but we need more information. Tell us what you can

[FrankA]

I wish I could get good such diesel mileage. I've never gotten better than 14 mpd. I've got 15,000 miles on mine so far and have yet to do any highway driving--hope it at least gets better for the highway.

[TXMAX]

I'm getting about 17.1-17.3 mpg in town so your 16.9 seems about normal. I have a SB CC so I have the small tank but your mileage doesn't seem too bad. Do you go watch them when they change your oil? If not I wonder if they are helping themselves to a few gallons? That would certainly explain what you are seeing. Let's see if I got 3 gal of fuel from 3 trucks a day I bet I would never have to purchase fuel. Just a thought. People do some strange things.

Abe

[7mmSTW]

The 16.9 was from the last good tank of fuel. We used to get around 600 miles from a tank. Give or take. Each time the oil is drained it seems to drop. We have kept it inside our shop at nights during the first bad mileage episode so I don't think anyone is stealing fuel. The low mileage was anywhere from 13 to 11. We changed diesel stations and that seemed to help some. I have wondered if someone could spray something on the MAF sensor that could cause the MPG's to go down. This could explain why the MPG's drop each time they handle it and gradually gain tank by tank. This thing has got me buggered. We have ran stanadyne in it with no change. I just put some synergin cetane additive in it. Have had lots of people refer that product to us around here. THanks again and keep it comin' we are tired of the fluctuation in mileage. If all it ever got was 13 that would be one thing but this is killin' us.

Later

Seven

P.S. '01 CC LB D/A 4x4

[dmaxstu]

Since I quit putting the pedal to the metal at every launch I now get 17.68 MPG mixed driving. However I've gotten as much as 21.5 MPG at 55 MPH in flat country on cruise driving all day that way,boy is that boring.So it is up to yourself drive gingerly and get good fuel mileage, drive normal you get still good fuel mileage for such a large vehicle,I used to drive a gasser van and was lucky to get 10MPG. 17MPG is good. Stu

[ 04-03-2002: Message edited by: dmaxstu ]</p>

[7mmSTW]

Just pulled it in the shop to check some stuff out. The MAF sensor looks clean to me. The air filter is clean also. It isn't even registering on the little tell tell guage. What else could this be? There HAS to be an explaination. I agree with all that 17 in town is GOOD. But when you drive it the EXACT same way, to almost the EXACT same places, it shouldn't suddenly drop to 11-13 for several tanks. This is the problem. It did it on the first oil drain and now the second oil drain. We will start to do it ourselves as soon as we can get a reasonable price on an oil filter around here. Let's face it, half of the US thinks New Mexico is another country and half of New Mexico doesn't realize that the eastern part of the state isn't Texas! smile.gif That makes it hard to get anything around here. :rolleyes:

Thanks again for the help

Seven

[More Power]

I don't see how changing the oil could affect the fuel mileage (unless you forgot to put oil back in :D ).

Has there been any change in performance or how the engine runs in general?

If you have gauges, how do the boost pressures and EGT's compare to when it delivered better fuel economy? These gauges can give you a visual clue as to what load the engine is seeing.

MP

[7mmSTW]

More Power, to answer your question no we don't have guages yet. I know we need them but things have slowed down around here for the time being. As for the power it seems there is little or no change. No smoke to speak of which seems funny to me that you can put that much diesel through the thing and not have any smoke. I have checked to see of there is any diesel leaking anywhere also. I agree that as far as an actual oil change goes there should be no reason absolutely whatsoever that could change the mileage. That being said, is there anything that a tech or anybody else could mess with that might do this. Twice in a row on the oil drain thing is just too hard to ignore. If this is a mere quinky dink as far as relationship with the oil drain is there some accessory that could cause this fluctuation in mileage? This is the reason why I wanted to talk to those who have been all over these engines. You guys have had more experience with all of the little sensors and other gadgets.

Thanks for your time.

Seven

[Tim B]

Is it possible that there's a fuel leak ? Maybe into the oil ? Seems like the oil drain dudes would have noticed that but still . . . .
And I know that wouldn't really account for ALL your missing fuel. Is it possible you've got a fuel leak at the tank or somewhere between it and the engine ?

Tim B.

[7mmSTW]

The fuel is definetly not leaking into the engine. I have been checking for that. There is no sign of a leak anywhere on the pickup that I can see. I have crawled under there several times thinking I will find something but I never have. This is pretty confusing to me.

Later

Seven

[7mmSTW]

Has anyone around here tried the Synergin Cetane additive? There is one company around here that buys it buy the 55 gallon drum for their work rigs. We haven't changed the rear diff fluid yet either. Where does it say that it needs to be changed after the first 500 miles of towing? I have wondered if this could be the problem but that still wouldn't explain the occaisional streaks of good mileage.

Thanks again

Seven

[7mmSTW]

Reason I wanted to talk to D-Max man is that I figured he could point me in the right direction for help. As I have said before, there are some people on here who get good mileage, some who get bad. If ours had started out getting bad mileage I wouldn't really expect much out of it. BUT, seeing how we started out getting good mileage and then it all of a sudden dropped says there HAS to be something causing this that can be fixed. Consideridering the fact that my '84 is getting better mileage than this D-Max we NEED to fix it. Our dealership techs wont do anything untill it sets codes.

Later

Seven

[mdrag]

7mmSTW,

You stated your mileage dropped severly after your first oil change, and the last full tank before the 2nd oil change was 16.9 mpg. What was the mpg before the first change? How many total miles and miles since the 2nd oil change?

[mark45678]

bad fuel??? Are you getting your fuel from the ssame place every time? and how often do you fill up? is the truck at the same pump faceing the same direction ever time? this all factors in to how full was the truck at each fill up! besides I am only getting 15~16 mpg.... 17 I would be jumping for joy!!!! but I only have my heavy foot to blame. :eek:

[Mic]

speaking of fuel.....when was the fuel filter changed last? :confused:

[7mmSTW]

OK, lets see if I can get all the numbers straight in my head again :eek: Here goes, before the first oil drain MPG's were as follows: 17+ in town, 21+ on highway, about 19 mixed, 19+ pulling our boat on highway, 14 pulling our 12,000lb 36' @70 plus MPH. It would go about 140 miles before moving off of full. After first oil drain it started coming off of full at 60 miles. For numerous tanks after that it went from a high of 14 to a low of 10. We changed fuel stations and it started getting around 15 and finally went up to 17 in town. Then we got our second oil drain and BOOM!! 13.8 We have only 2,000 miles on this fuel filter and the old one looked clean. ME CONFUSED!?!? :confused: :confused:

Later

Seven

[Amianthus]

How about some of the other things I, and others, suggested you look at? How did they come out?

I know you're gonna hate this, but look at your fuel filter. Maybe even change it (shouldn't cost that much). Switch fuel stations. Maybe the station you switched to is using a richer winter blend and that's hurting your mileage. Keep looking.

[7mmSTW]

Amianthus, I guess it came out wrong in my last post but we have changed both the fuel filter and stations. There was no change with the new filter and about a 1 MPG increase with the different diesel station. Guess the old one was watering down his whiskey a little.

Later

Clay

[Turbo Al]

I stongly suspect a draging brake or drive train problem. Does it roll easily on level ground? My last severe MPG drop was because of a defective rear end, pulling the cover was all that was needed to confirm this. Do you have the one piece drive shaft?

[7mmSTW]

We have the original driveshaft. What exactly is the deal with the one-piece? The earlier posts that I have seen they were putting them on the short beds. Is this something we should look into with our long bed? I was doing a Co-op with the New Mexico Game and Fish all summer and fall of last year so it was hard for me to keep up with the DP during that time. I don't think that the driveshaft could cause a sudden drop in mileage though could it? The rearend would definetly be worth looking into though. What exactly happened with yours? Questions Questions Questions.

Thanks again

Seven

[7mmSTW]

How exactly does the one-piece driveshaft increase MPG's?

[Turbo Al]

My truck 2001 ext cab 4x4 long box - came with the one piece drive shaft. I don't think the one piece drive shaft would increase MPG just that there were some posts on the two piece "binding" & "launch shutter" which can't be good for MPG.
Do a seach of drive shaft and see if you qualify for the one piece.
The rear end I had trouble with was an Eaton limited slip and it was a cluch pack that came apart.

[7mmSTW]

Thanks Turbo Al, ours is a Crew Cab long box so I don't think we have the driveshaft problem. This tank isn't going too good either but I will keep everyone informed of what's going on. One things for sure, I don't know if it's the Synergin or what but this thing is running like a bat outa Hades. It is running really strong without really asking for it. Don't think that a lead foot is to blame for our MPG's because it is being driven the same as when it was getting good MPG's.

Later

Seven