refueling

Discussion in 'Trucks [ Eighteen Wheelers ]' started by 1999blue, May 2, 2009.

  1. 1999blue

    1999blue Light Load Member

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    Dec 27, 2008
    quartzsite,AZ
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    I have a FLD 120 freightliner,, two 100 gal saddle tanks, After topping off does the fuel cross feed and stay balanced ,,, or does the engine draw off both tanks while running. I have no left/right electric switch.
     
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  3. Peanut Butter

    Peanut Butter Road Train Member

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    Nov 24, 2007
    PO Dunk Idaho
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    It Draws off both tanks while running, both tanks should stay at the same level unless the check valve is bad or something else is wrong.
     
  4. Heavyd

    Heavyd Road Train Member

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    Feb 4, 2009
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    With dual tanks there is no "check valves" or "equalizer" valves. The draw and return line from each tank only go to "T" fittings. The tanks stay equalized based on physics. If you ever look inside your tanks they have pickup and return tubes that run all the way to the bottom of the tank. The way it works is the pressure of the fuel is higher at the bottom of the tank than at the top, just like water pressure at the bottom of an ocean is higher compared to the surface. With the tanks, obviously the difference in pressure is very minimal. If you have one tank that is full and one that is near empty the fuel in the full tank will be under higher pressure at the bottom than the fuel in the near empty tank. We all know this because if you drain oil from the oil pan the oil rushes out very fast at the beginning because of the higher pressure, once the oil is almost drained, there is little pressure and the flow slows down. Anyway, back to the fuel. What makes the pressure is basically the weight of the fuel on top of itself, the more fuel the heavier it is. The full tank has more pressure at the bottom than the near empty tank and so the engine sucks fuel more from the full tank because the higher pressure helping the engine suck it. When it comes to the return, it is harder for the return fuel to go to the full tank because the fuel pressure is higher. With the fuel pressure being lower in the near empty tank the return fuel has an easier time going to the tank. This is how the tanks maintain an equilibrium. This is not to say you only need to fill one tank. Since the pressure difference is so small it isn't that precise.
     
  5. 112racing

    112racing Road Train Member

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    Nov 30, 2008
    pocono's, pa
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    i fill one tank all the time and the fuel stays level in both tanks also my right tank is split leaving only 40 gallons of fuel on the right side with a 120 on the left and they always have the same level
     
  6. bulldozerbert

    bulldozerbert Medium Load Member

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    Oct 5, 2008
    Ontario
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    So, are you saying you can top one tank up, or should you always fill both tanks at the same time?
     
    Last edited: May 4, 2009
  7. 112racing

    112racing Road Train Member

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    Nov 30, 2008
    pocono's, pa
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    like i said i only fill one tank most of the time and after a few hours they will level themselves out you do not have to fill both tanks
     
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