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Cummins 400 Big cam 3

Discussion in 'Trucks [ Eighteen Wheelers ]' started by smugglinggoods, Aug 29, 2011.

  1. Aug 29, 2011 #1
    smugglinggoods

    smugglinggoods Light Load Member

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    Maybe someone can point me in the right direction. When I start my truck the engine starts easily but immediately after it starts the rpms go up then come down to where sounds like it will die then maybe back up. If I give it some throttle for a bit then once I let off it runs smooth. Does this when its warm or cold almost anytime I start it.

    So what should I look at?
    Thought about changing my fuel lines first

    What do you think?
     
    smugglinggoods, Aug 29, 2011
    smugglinggoods, Aug 29, 2011
    #1
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  3. Aug 29, 2011 #2
    bender

    bender Road Train Member

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    I've seen this happen when the fuel crossover o-rings are seeping slightly and allowing air to enter the fuel passages in the heads. Replace the 4 o-rings on each crossover and see if that straightens it out.
     
    bender, Aug 29, 2011
    bender, Aug 29, 2011
    #2
  4. Aug 30, 2011 #3
    Diesel Dave

    Diesel Dave Last Few of the OUTLAWS

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    I forgot what year it was when cummins went with the variable timing deal, but when it needed replacing , it would smoke and run rough at start up till it warmed up. It was next to the fuel pump.
     
    Diesel Dave, Aug 30, 2011
    Diesel Dave, Aug 30, 2011
    #3
  5. Aug 30, 2011 #4
    smugglinggoods

    smugglinggoods Light Load Member

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    Thanks for the info. I will try the cross over lines. I have had the heads off recently so could be something there.
    Diesel Dave could you try to be a little clearer on your thoughts. Not to sure what you meant I should look at? Engine is a 84' by the way.

    Thanks for the help
     
    smugglinggoods, Aug 30, 2011
    smugglinggoods, Aug 30, 2011
    #4
  7. Aug 30, 2011 #5
    Diesel Dave

    Diesel Dave Last Few of the OUTLAWS

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    I just reread your post, It sounds like your loosing the prime in your fuel lines. How many filter's do you have, do they have the o-ring in the center of the filter, are the filter/s full of full before you start it ? check them next time b4 you start the truck.
    What I was refering to earlier was the variable timing sileniod, when they go bad the truck will smoke bad and run rough unitl it warm's up.
     
    Diesel Dave, Aug 30, 2011
    Diesel Dave, Aug 30, 2011
    #5
  8. Aug 30, 2011 #6
    smugglinggoods

    smugglinggoods Light Load Member

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    Thanks again
    I will check the filter in the morning. It just has the one fuel filter and its just the screw on canister type

    Thanks
     
    smugglinggoods, Aug 30, 2011
    smugglinggoods, Aug 30, 2011
    #6
  9. Aug 31, 2011 #7
    Shade Tree

    Shade Tree Light Load Member

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    Check the fuel line fitting seats to ensure there isn't any burrs on the seat that allows entry of air in fuel system. What you describe is similar to losing prime.
     
    Shade Tree, Aug 31, 2011
    Shade Tree, Aug 31, 2011
    #7
  10. Aug 31, 2011 #8
    smugglinggoods

    smugglinggoods Light Load Member

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    I pulled the fuel filter this morning and it was only 3/4 full. What should I look at here what would cause this? Thanks again for the help guys
     
    smugglinggoods, Aug 31, 2011
    smugglinggoods, Aug 31, 2011
    #8
  11. Aug 31, 2011 #9
    bender

    bender Road Train Member

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    When the engine is shutdown, some fuel from the filter siphons back to the tank and this is normal for a PT fuel system. The filter will fill quickly when the engine is started.
     
    bender, Aug 31, 2011
    bender, Aug 31, 2011
    #9
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