My dd15 Cascadia125 MPG drop too much!Can't find any reason!

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Leo717, Apr 10, 2024.

  1. Leo717

    Leo717 Bobtail Member

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    Apr 10, 2024
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    I have 2017 Freightliner DD15,AT trans, 850K mileage,i usually haul load between 5k-20k lbs,never heavy load,driving 70-75 mph,i always used blue DEF to fill.So mpg around 8.0, But after i parking this truck 2.5 months,mpg going down to 7.0, no any code on screen,no engine light,no any problem checked on computer.
    Since I am usually very sensitive to my truck, I can clearly detect that there is now a resistance when coasting downhill, because before on medium or small downhills, I did not need to give diesel to maintain a certain speed, but now, other conditions all same(like load weigh,ambient temperature etc.) I need to put on the diesel when going downhill to maintain a certain speed. If not, the speed will gradually decrease while going downhill.
    The reasons I thought at first have been eliminated, such as ambient temperature, clogged engine air intake filter,brake incomplete separation,over fill engine oil.And I also suspect that the fuel injector is clogged,but after used Lucas Fuel Treatment running 20k miles,noting change.Trailer problems can also be eliminated.
    The only thing I can't rule out is a fuel injector failure, but the engine cylinder pressure all above 90%. If the fuel injector fails, then where does the driving resistance come from?
    Please help~~~very confused :(:(:(
    .
     
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  3. LameMule

    LameMule Road Train Member

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    I've heard of soot buildup in them things causing problems. I assume it has a variable geometry turbo, they can soot up or function poorly as well. Fairly easy to look at the intake to see if the egr is full of soot.
     
  4. xsetra

    xsetra Road Train Member

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    The one box could be packing up with soot also, restricting exhaust flow.
     
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  5. shatteredsquare

    shatteredsquare Road Train Member

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    when is the last time you greased the clutch release bearing
     
  6. Sm0kie

    Sm0kie Bobtail Member

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    May 15, 2024
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    He said it was AT trans?
    Pull your delta p sensor. Check venturi ports for soot build up/plugging.
     
  7. uncleal13

    uncleal13 Road Train Member

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    They still have a clutch unless it's an Allison.
     
  8. uncleal13

    uncleal13 Road Train Member

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    If the truck has sealed s-cam tubes they may be drying out so the brakes are not fully releasing. If it has that style they can be greased.
    I would suspect a wheel end problem of some sorts seeing as it is dragging as you say.
     
  9. Sm0kie

    Sm0kie Bobtail Member

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    I'm aware but being a 2017 it is most likely a DT12 which doesn't have a greasable throw out bearing
     
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  10. shatteredsquare

    shatteredsquare Road Train Member

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    it could be a lot of things but the OP description sounds exactly like my eaton 10 speed auto throw out bearing screaming for grease. loaded heavy it's hard to tell, load momentum masks the symptom, but after delivery, pulling the empty, when you let off the throttle there is a unreasonable amount of highly noticeable deceleration, due to clutch release bearing not disengaging from engine. a few pumps of grease into clutch release bearing, no more deceleration on throttle release, it coasts forever just like if it was in neutral. but could be brakes, hubs. if it was brakes and it didn't get better or different over time, there would be smoke at some point. if it was a hub, for there to be that much friction would mean also a lot of heat, you would be able to see smoke or feel heat if it didn't get better through the day. but clutch release bearing not letting go of the engine will do the toddler-attached-to-pants-leg symptom without any other noticeable symptoms. either way, something needs lube!
     
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