C12 Runs Rough When Cold

Discussion in 'Kenworth Forum' started by Veeg378, Nov 11, 2019.

  1. Veeg378

    Veeg378 Bobtail Member

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    Apr 10, 2017
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    Buddy of mine has a Kenworth with a C12. When ambient air temp is below 50 degrees Fahrenheit the truck shows 9-13 on fuel restriction gauge. Summer time it’s on 0.
    All of the following have been replaced; fuel filters, suction and return fuel lines, fuel filter housing for water separator and fuel pump.
    At idle the truck has 70psi of fuel pressure and at high idle its at 90.
     
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  3. Lowboy456

    Lowboy456 Light Load Member

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    Mar 14, 2017
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    I saw a Freightliner FL70 do this. It had some weak injectors.
    If you can do a cylinder drop test it might help to determine which cylinder it is.

    We also had a 98 KW T800 that wouldn't make it around the block because of fuel restriction and smoking white like crazy which is a sign of being starved for fuel or too much air.

    Turns out the line from the filter back to the tank needed blowing out good. It was idling at 4 on the restriction gauge and would go way high when revving the engine.


    Many times diesel will leave behind tiny particles in 90 degree elbows or filter housing fittings that stick together and get very hard.
    When that happens that stuff has to be picked out with an awl or something. You can't spray a solvent on it and get it out.
     
  4. Chubby Fly

    Chubby Fly Medium Load Member

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    Nov 25, 2019
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    Should show more than zero on restriction gauge even with new filter and new lines. Mine shows high readings sometimes. You have to tap the gauge. Those #### things stick a lot. Next time you trucking and gauge reads high, give it a few taps with finger and watch how the thing moves down instantly
     
  5. Lowboy456

    Lowboy456 Light Load Member

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    Mar 14, 2017
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    My new fuel filter reads higher than the one I took off which was a surprize to me. Its like 4 cruising down the road maybe just under when backing off a bit downhill.
     
  6. Chubby Fly

    Chubby Fly Medium Load Member

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    Nov 25, 2019
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    Yeah happens to me too. I pay that no mind. Usually I just swap out when the gauge goes into the red
     
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