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Any harm in running both internal and external lift pumps?

Discussion in '5.9L 12 valve Cummins' started by bob_442, Nov 30, 2020.

  1. bob_442

    bob_442 mad scientist

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    This is for a mid-90's 6BT with a P-pump. Currently the vehicle has two tanks, each one with an internal fuel pump. There is a Summit regulator limiting the pressure to about 35 PSI.

    One of the pumps is having real trouble maintaining pressure. At first it seems to make 35 PSI, but after a few miles on the road, it can drop to 20 PSI. These AFAIK are stock chevy pumps for a 454 in the tanks. The other pump seems to keep the pressure up OK. You can absolutely tell the difference with the engine note and the throttle responsiveness when switching tanks, and you can see it in the in-cab pressure gauge. I've driven several thousand miles since i noticed what I've described above; I'm not seeing any changes over time.

    OK, so I know what the right answer is -- get rid of the internal pumps, install an external one, and carry a spare. Now that I got that oughta the way, my question is, is there any harm in installing an external lift pump between the fuel tank selector valve outlet and the regular, to help get the pressure up to a steady 35 PSI (leaving the internal pumps powered-up and in-place)?
     
  2. Philip1

    Philip1 Supporting Member Supporting Member

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    The main problem i could see would be the pumps fighting each other
     
  3. bob_442

    bob_442 mad scientist

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    I wondered about that. Like maybe the second pump -- the external one -- sucking fuel through the internal one faster than it can push the fuel. Or, the internal one pushing fuel into the external pump faster than it wants to receive it. Since the external one is likely the higher capacity pump, this is unlikely, though possible I suppose.
     
  4. bob_442

    bob_442 mad scientist

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    Answering my own question here...I found this web page to be *very* interesting -- especially the last paragraph.
     

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