6nz Fuel Mileage

Discussion in 'Trucks [ Eighteen Wheelers ]' started by D389, Mar 24, 2013.

  1. D389

    D389 Light Load Member

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    Mar 24, 2013
    Texas Panhandle
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    The truck is an '01 379, 740k, C15 6nz 475 reflashed to 550 according to prev owner, super 10, 3.55 rears, and 245/85-22.5 tires I believe. We are looking to get more than 4.3-4.4 mpg out of this truck. It is used to haul hay in Tx, Nm, & OK, fairly flat states, mostly grossing 75-82k. The only modifications done to it are a fuel temp harness & straight mufflers. I know pulling big round bales is like dragging a parachute, but it gets the same mileage going 62 as it does at 75+ which doesn't seen right. Also stays the same pulling squares which don't hang off the sides. The only thing I noticed that didn't seem right was the exhaust temps run 900-1100f post turbo most of the time at speed 65+, loaded or empty. The turbo makes 28psi and feels tight. Im leaning towards the gearing being wrong 1525@70 or the unknown tuning in the ecm. It does smoke some at low boost but not much. What are you thoughts on this? BTW this site is great, wish I had joined sooner!
     
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  3. Red Rocket 1964

    Red Rocket 1964 Light Load Member

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    Jan 31, 2013
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    Is the MPG mentioned calculated or off the in-dash? I would start by having the ECM returned to stock or tuned by someone you trust. No telling what is in there now. 75+ MPH with a 379 is not going to result in good MPG no matter what. I might consider 11r22.5 tires when you need drives, that should put the RPMs at 1300ish at 65. My 387 mbn c-15 (with 6nz tuning 550hp +injector codes and fuel temp wire) would do 7mpg driving at 65mph max. Is the wastegate blocked? you should have 32psi boost.
     
    h50 Thanks this.
  4. bubbanbrenda

    bubbanbrenda Road Train Member

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    Feb 27, 2011
    Middletown,Oh.
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    I think you've got at least 2 problems here. If you're getting 9-1100 deg especially empty, you've got something going on in the turbo/cac. Your biggest MPG enemy here is the extra short 22.5's and the 3.55's. Next time you buy tires buy tall 22.5's that will help the MPG's. Also I don't know the exact final drive in a super 10, but assuming a .74 final drive the math isn't adding up, You either have tall 22.5's already or you have 3.08 rears to go 70 @ 1525
     
  5. Oscar the KW

    Oscar the KW Going Tarpless

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    May 19, 2011
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    Sounds like you need to have your cac checked.
     
  6. D389

    D389 Light Load Member

    99
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    Mar 24, 2013
    Texas Panhandle
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    The tire size is actually 285/75-24.5, I was thinking of one of the trailers on the first post. I hand calculate the mileage every time I fill up, this truck doesn't have the Cat ID on the dash, wish it did though. The wastegate isn't blocked. Could an exhaust sleeve going through the turbo cause all the heat? I noticed it doesn't have the shrill whistle out the stacks like it did when I started driving it in December.
     
  7. little cat 500

    little cat 500 Road Train Member

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    Jan 17, 2010
    Abbottstown PA
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    sounds like the cac is cracked or some other boost leak
     
  8. D389

    D389 Light Load Member

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    Mar 24, 2013
    Texas Panhandle
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    Just got the truck out of the shop and they replaced a bad injector and fixed the cac. They also said the turbo will need to be replaced. What would be a good upgrade that will help some with mileage and airflow and still behave like factory?
     
  9. Blind Driver

    Blind Driver Road Train Member

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    Aug 7, 2006
    New Albany, IN
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    I wouldn't want my engine to behave like factory. I want it to be better than factory ;)

    "Mr. Haney...paging Mr. Haney. You are needed in operating room #1"

    I used to drive a '03 379 with a 550 Cat 9 speed with I think 3.08 gears. . It would get 5.0 at 75 (1600 rpms) on the cruise and 6.0+ at 67 with some moderate foot applied just before the hills. The engine would really quiet down at 75. I guess that is the way it was tuned.
     
  10. mark mechanic

    mark mechanic Bobtail Member

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    Mar 4, 2013
    Adelaide Australia
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    plenty of optsions out there bullydog borgwarner etc but good balance fuel injectors more inportant start point and good tune up ie boost leaks check at 25-30 psi at lest distcharge o rings on turbo mostly missed try two sections of wire 316-532 diameter under clamping wings to lock down o rings dont wear out after this.
    we use injectors from 'adelaide' "australia" from e.diesel great fuel milage realy good power with minimal smoke if any, and they can set up higher fuel deliverys if required they have supplied 6nz and nsx acert units for us many times with repetable results if you message me iwill send you a e mail address
     
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