About two months ago my truck suddenly went from 18-20mpg to 15-16mpg and has since fallen even more to 13mpg!! This is my first diesel so I came here and did research to see what I could do to fix it. I replaced every filter I could, had it aligned, changed the tires (they were due anyway), looked carefully for cracked or leaking lines. Had a minor oil leak from the filter. Fixed now. Nothing has made any improvement. There is NO loss of power at all. Turbo still kicks butt. The thing is a beast when it tows. I asked the local shop if they had any other ideas and they said that the valves probably need adjusting. Their opinion is that they should be done every 30,000 miles but I see on here that Cummins doesn't spec it until closer to 130,000 miles. My truck is at 127,000miles. But with no power loss doesn't seem the valves would be an issue even if they do need an adjusting now.

It does have some electrical gremlins- nothing significant. The interior lights occasionally won't shut off and the door open bell chimes upon acceleration (these happen together). It goes away in a few days. It throws the code (can't remember it, dang!) that seems to be pretty common- saying that the cam alignment is off but it's is probably just the wiring. It goes away in less than a day. Electric door locks are wonky. Weird stuff like that doesn't seem like it should or could be a factor.

However do any of you that have had diesels and these Dodge's in particular have any ideas what might be up or what I should try next? I'm about out of ideas! THANKS in advance for any help you can send my way!

 

Winter grade fuel and engine not getting to running temperature can cause lower mpg #'s.


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Thanks. I buy most of my diesel in the lowlands where additives aren't required and I commute in this truck so I know it does get to running temperature. Unless you mean that driving it BEFORE it reaches running temp is the issue? That I'm guilty of!

Weird is that one tank included me driving it up our mountain (gain of 6000ft) with a big load of hay- 13mpg. The next tank was mostly freeway driving at 65mph and no loads- 13mpg.

 

Ok, so this is a little scary. No one has any suggestions? All of you with great diesel experience are stumped? (well, or busy. I get that!) Does this mean I'm screwed and bound to spend a fortune at the local shop trying to find out what's wrong?

 

A far as I know winter grade diesel comes from the refinery certain times of year and mpg can go down. Like i said if the engines dont get to running temp they may consume more fuel. I know this winter I can drive my 05 for 30 minutes before it even gets close to normal temp.

Not saying its not something else but just some thoughts on why your milage is down some.


Might be something causing drag on the engine. Is it rolling fine? Could it be a pulley issue or something on the serp. belt causing drag. I had a AC compressor go bad on my 95 Ram one time and caused serious drag on the engine and it consumed more then normal fuel.


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I took it in and had the shop put it on the computer. Nothing. Everything looks good. But they insisted it needed a cleaner run through the injectors and that I couldn't buy the quality of product that they could get. Since I am new to diesels I told them to go ahead even though it sounded like hogwash. So $130 they poured a bottle of stuff in my tank, sprayed some more stuff on the injectors, and told me my air filter was dirty (nope- just changed it 6 weeks ago). Hoping it will make a difference but not expecting it to.

 

No offense but I think they took you for a $130 ride. Coulda bought another tank of fuel!
Has the elevation you're driving your truck changed?
Mine gets noticeably worse mileage at low altitude. Not 4-5 mpg worse but about 2 mpg worse. Maybe it's my imagination or driving conditions but averaged about 18 mpg at 6-8kft. 2 yrs in Seattle now more like 16-17 mpg even on the highway. Think it pushes more fuel to go with more air it sees here?

At 130k mi maybe a bad injector or 2? Not enough to flood the crankcase but enough to dump a little more fuel?
Good luck figuring it out.

 

Well, whatever crud it was they used it does seem to have worked! MPGs are back up to 18-19 on the mountain roads!! If they stay here the $130 will be recovered in just a couple of tanks of fuel!! :-)

 

Adjust the valves they need it and that can help.

 

Ok, first tank 19mpg, next tank 13mph, next tank 18mpg, next tank 15mpg. What the heck? I did notice that I'm getting a lot more blacking on the bumper above the tail pipe. Is that a symptom of something I'm missing?

Guess it's time to adjust the valves. Is that a job someone with decent but not advanced skills can do relatively easily and effectively?

 

I have experienced this twice before and am now seeing this again. If you have gauges and watch them you soon discover what is normal. When this happens to me, I also see a slight rise in EGT of 50-100 degrees when driving flat and level at about 60 mph In all of my experiences it has been bad universal joints adding rolling resistance. There are also other clues as they get worse, but loss of milage is usually the first clue.