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Hey diesel techs

Discussion in 'General' started by tophyr, Nov 23, 2022.

  1. tophyr

    tophyr Grid Filler

    My RV has a Cummins ISL in it. 9.8L, rated at 400hp. The RV weighs about 36k lbs, and gets 5.5mpg on flat ground at 60mph. Pulling a stacker, which makes total weight 47k lb and total aero area 8' x 13.5', it gets about 4.0mpg @ 60mph.

    Talking offhand with random truckers at fuel stops, who have basically the same aero, they average around 8mpg at 40k going down to 5mpg at 80k.

    So, it sounds like I'm getting about half the mpg I should be. It wouldn't surprise me to find out the motor in this rig isn't in the best health - it works and is reliable and etc, but if it's in the same condition the rest of the RV is then it definitely needs some TLC.

    Aside from changing the oil, what maintenance items can I do to improve mpg on this thing? It's not blowing black smoke or anything, and the turbo isn't giving any obvious signs of trouble (still makes 30psi, etc). I've heard dirty/clogged injectors can cause mpg problems, but I don't honestly really understand how - if they're clogged, it'd use less fuel and simply have less power, right?

    I think part of the problem is that I don't really understand the common-wisdom relation of "more power -> more fuel efficiency"... that seems backwards to me.
     
  2. Rising

    Rising Well-Known Member

    Have you checked the exhaust? Maybe there is a snake stuck in it.
     
  3. Rising

    Rising Well-Known Member

    I think it is more about efficiency at lower throttle settings; or more power for a given throttle position. Dirty injectors can result in more smoke from the exhaust.
     
    tophyr likes this.
  4. zertrider

    zertrider Waiting for snow. Or sun.

    ISL is 8.9L, not 9.8L.

    That's all I've got for you. Talk to Brazels RV Performance out in Washington state.. They may be able to help
     
    tophyr likes this.
  5. CRA_Fizzer

    CRA_Fizzer Honking at putter!

    You have how many gears? They have how many... What is your rear end ratio?
    You have a pusher? If so, you need to compare to a cab over as you are both boxes in the front.
     
  6. A. Barrister

    A. Barrister Well-Known Member

    Do you have the same gearing as the other people you have talked to?
     
  7. iagsxr

    iagsxr Well-Known Member

    I kind of wonder about transmission and gearing as well. That and if it has a restrictive exhaust for noise.

    The other thing vs. a semi, a bumper hitch trailer is way less aerodynamic than a gooseneck or fifth wheel. Especially if you have an extended "RV" tongue on your trailer you're getting a huge amount of air deadheaded against the front of the trailer.

    Not exactly the image I was looking for but you get it...

    21f30f4cdd4413eeb60fa29fe4555b30.jpg
     
    Last edited: Nov 23, 2022
    cbush likes this.
  8. mastermind

    mastermind camping in turn 2....

    In our DP we have a CAT 3126, 330HP... coach is about 29,ooolbs, and we typically pull an 8.5x20' Haulmark trailer loaded to around 6500lbs or so.

    Actually blew the motor last May going up I-24 towards Nashville (just under 300k miles on it) but before that it was running good and giving us about 9.5mpg (65mph) without the trailer, and about 8.5mph with the trailer.

    New motor seems to be getting about the same mpg, but I haven't measured it yet. Waiting to get at least a few thousand miles on it. New motor jumps off the line a lot nicer - old motor was tired by comparison.....
     
    tophyr likes this.
  9. TWF2

    TWF2 2 heads are better than 1

    That does not sound to far off, maybe 1-2mpg. You got a lot of weight there.
    I had cummins 300hp and was getting double of that (10-11mpg) but weight was around 30k.
     
  10. tony 340

    tony 340 Well-Known Member

    47k gross weight all in?

    That motor should pull that with ease and knock down 8mpg+
     
    tophyr likes this.
  11. tophyr

    tophyr Grid Filler

    Yeah, 47k loaded on the CAT scale
     
  12. Mongo

    Mongo Administrator

    My vote is gearing is the biggest difference.
     
  13. tophyr

    tophyr Grid Filler

  14. tony 340

    tony 340 Well-Known Member

    Guessing your boost was just a tad low :crackup:
     
  15. tophyr

    tophyr Grid Filler

    what's crazy is the boost gauge reports around 30psi. so now i'm dubious of that sensor too, heh
     
  16. Mongo

    Mongo Administrator

    Good excuse for an upgrade :D
     
    tophyr likes this.
  17. tony 340

    tony 340 Well-Known Member

    You still may have lots of boost, but shit isn't flowing the way it should.

    Is that the exhaust manifold or exhaust pipe itself
     
  18. tophyr

    tophyr Grid Filler

    looks like just a pipe. i'm gonna keep poking around a little bit to see if it's something i could take off and take to a shop to have fabbed. i don't have a ton of time however, i'm *supposed* to leave for chuckwalla tomorrow and then go from chuck straight to daytona. will be trying to see how much of this i can do on the road i suppose.
     
  19. pickled egg

    pickled egg Tell me more

    That must be how Tammy got in…
     
    tophyr likes this.
  20. ducnut

    ducnut Well-Known Member

    You should probably check all your intake hoses and intercooler (< pressurize it) for leaks, as well. Make sure your fuel and air filters are clean. K&N does offer an air filter.

    I’d be running a fuel additive like EDT. Measure it out in a ratio rite and add it, each time you fuel. It’ll clean out your tank, pumps, lines, and injectors, so an extra filter is advisable. It’ll blacken a filter with garbage, in the first couple weeks. After awhile, that’ll stop, as the system gets cleaned out.

    https://www.hotshotsecret.com/diese...SZxpXCM-9P0rNJofi-4TSRGPAFz53DphoCLtYQAvD_BwE
     
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