7.3 idi c6 mpg

Discussion in '6.9L IH & 7.3L IDI Diesels' started by crr31c, Nov 7, 2018 at 11:04 PM.

  1. crr31c

    crr31c Registered User

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    Help!! New to the forum. I have an 88 F350 dually 7.3 no turbo, c6 3.55 gears. Only getting about 9 mpg unloaded. Newly built engine new injectors, older IP but in good shape, timed perfect with ferret meter, turned down. Fires right hip cold or hot with out throttle. Only a little black smoke when i get in it. Otherwise burns clean. Mostly highway. 60 mph about 3000 rpm, 650-700 deg. On the pyro. I know these trucks aren't known for fuel mileage. But really? 9 mpg unloaded. I'm only gonna get like 6 mpg hauling my travel trailer. Any help would be greatly appreciated.
     
  2. riphip

    riphip Supporting Member Supporting Member

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    Welcome to the forum!!

    How many miles on the IP??

    That older IP probably not in as good of shape as you think. Try replacing fuel filter filled with Tranny fluid & run for about 5-10 minutes then turn off. Let set for 12 hours, crank and run it like you stole it. Cleaning out the IP.
     
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  3. Hydro-idi

    Hydro-idi Full Access Member

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    My rig was getting a consistent 16 mpg (hwy) with C6 & 3:55’s. Went up to 18 once I put the 3:08’s in.
    It’s probably your injection pump. New sticks and old IP never go together.
     
  4. crr31c

    crr31c Registered User

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    Thanks for the help. Really trying not to spend the $$ on a new pump. I have so much in this truck already. And it is so loud I'm not sure I'm gonna be able to get my wife to travel in it. So.....

    riphip, do i just use regular dexron tranny fluid? And do i fill the new filter completely with the tranny fluid?
     
  5. homelessduck

    homelessduck Full Access Member

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    My cc 4x4 c6 4.10's gets about 12 mpg.
     
  6. Clb

    Clb Another old truck Supporting Member

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    My 88 sucked...m\t
    new eng. Sticks, junior moose c-6 4:10 with a taller tire.

    The 93 gets a best of 14 m\t

    edit m\t = empty
     
    Last edited: Nov 8, 2018 at 10:05 PM
  7. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    Doesn’t matter what kind of atf. Have you adjusted the vrv?
     
  8. jayro88

    jayro88 Full Access Member

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    My '88 e250 with a c6 and 3.54's can get in the 14-16mpg range mixed driving if I keep the cruise set in the 60-65mph range. I am sure your truck is heavier than my van, but trucks usually do a little better mpg wise than the vans......but I get better than you are stating towing my 7000lb travel trailer with my egt's hanging out in the 900-1100* range.

    It sounds like your EGT's are a little high for the speed you are traveling. Mine fluctuates due to hills and all that, but 60-65mph usually has me in the 550-650* range.

    A lot of things can play into MPG. I would start by making sure you don't have a break dragging. Tire type and psi matters. Driving style.

    I agree that your IP could be affecting it. Also, how new built is the engine? I didn't do the RPM calculation, but are you sure you have 3.55's?
     
  9. crr31c

    crr31c Registered User

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    Yes. Trans shifts fine
     
  10. crr31c

    crr31c Registered User

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    The engine has 200 miles on it. I considered a brake issue my self. going to check this weekend. Because it does seem like truck loses speed excessively when i let of the gas to coast. It has 3.55 for sure. I put them in. It had 4:10's, and i swapped them. Another question, how do you hear yourself think at 65mph. I've put the quietest muffler they make for this truck on and filled the cab and engine bay with dynommat and carpet padding, and I'm still roaring in the cab at 60mph.
     
  11. jayro88

    jayro88 Full Access Member

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    It does sound like something is dragging. I also added quite a bit of sound deading to my van. It helped quite a bit, but I still have more wind noise than I would like. I need to replace the door seals still.

    The biggest thing that improved the noise on my cab was when I added my DNE2 auxiliary O/D. With it I can get in the 18-20mpg range if I it is all hwy and the conditions cooperate.

    What kind of torque converter do you have? A low stall diesel one could reduce slip and increase the mpg some....also drop the rpms slightly.

    What is your timing set at?
     
    Last edited: Nov 8, 2018 at 2:47 PM
  12. renjaminfrankln

    renjaminfrankln Full Access Member

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    I had an 85' with a C6 for a short time. Pretty truck, but I knew after about 3 days of ownership I was gonna have to sell it.

    I sold it and bought a later model with a ZF5.

    Buying an aftermarket OD is an option, or swapping the transmission is an option. You'll have better luck finding a used OD for a 2wd vehicle I think.
     
  13. crr31c

    crr31c Registered User

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    Sorry about not getting back to you guys. Crazy at work. Timing is 8ish btdc at 2000 rpm. Went by all the info i was able to find online. Torque converter is stock
     
  14. jayro88

    jayro88 Full Access Member

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    Timing sounds about right.

    The stock torque converters stall is a little high for diesels, but it wouldn’t make more then 1-2 mpg.....maybe. I watched my EGTs on the way home today. On flat ground with the OD off I was under 600* cruising at 65mph. That is being very easy on the throttle inputs too.

    I know with a dually it may be a little higher due to the 2 extra tires and the added weight, but no way you should be getting under 10mpg at 65mph. I would think you would be closer to the 14-16mpg range. Something is definitely up.


    Sent from my iPhone using Tapatalk
     
  15. crr31c

    crr31c Registered User

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    Yeah starting to wonder about bearings or something. Deff seems like i have some rolling resistance the more i hash it out on the forum. I did the math and should be around 2350 rpm at 60 not at the nearly 3000 I'm at. So going to look into that this weekend. Thanks for all the help.
     

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