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Fuel feed pressure or pressure/vacuum gauge

Discussion in '6.9L IH & 7.3L IDI Diesels' started by Vern, Feb 18, 2020.

  1. Vern

    Vern Full Access Member

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    I don't understand something. Why is it a pressure gauge behind the IP is a valuable tell-tale? Don't you want to know if it's sucking too hard, primarily? I mean, what is zero pressure indicating? No differential pressure, right? It's not starving and things are flowing, right?

    There's a vane pump in there but not a good one I understand. So, that vane pump, when really pulling fuel is still happy at 0psi. it isn't even sucking. So what is the utility of the pressure guage?

    see, what happens if there's a restriction. Your diesel gels, let's say. What is that pressure gauge going to do to help you pin-point fuel delivery issues?
     
  2. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    To me, "behind the IP' means "after the IP". Common fuel pressure gauges won't even stand a chance of reading the fuel pressures that come out of the IP. I think your mean before the IP, right? This is to monitor the inlet pressure to the IP. another way to say it is that it monitors the pressure put out by your lift pump. The lower inlet pressures to your IP cause your IP timing to be retarded. While your IP will pull fuel on it's own, the performance will be down compared to the performance that you will have with say 5 or so PSI coming into the IP.
     
  3. Vern

    Vern Full Access Member

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    Why? in front of the IP are the injectors.
    Transfer pressure is what we are talking about.
    I'm not so sure because of the internal regulation of pressure/relief.
    I'm not so sure because of the internal regulation of pressure/relief.
     
  4. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    In the fuel system, the fuel starts in your tank, goes through the fuel lines, through the lift pump, through the filter, through the IP, to the injectors, then, finally, what isn't used goes back to the tank through the return lines. The injectors are AFTER the IP. Also, the radiator is in front of the IP while the injectors are behind it. As for the inlet pressures, you'll have to have someone who knows more about the internals of an IP than I do to get a better explanation of this that I can. maybe Macrobb or Wes can chime in to help.
     
  5. Vern

    Vern Full Access Member

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    Heat goes bottom to top

    fuel goes tank to engine; flow forward is my view.

    thanks, I guess I can refine my question to does the transfer pump require positive pressure.
     
  6. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    The transfer pump does not require positive pressure, it is however calibrated with five psi of inlet pressure on the test stand so that should be duplicated on the vehicle for the best economy and performance. The vane pump is far from a poor design, I have taken it as far as 400cc of fuel so far without needing to run an external transfer pump.
     
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  7. Vern

    Vern Full Access Member

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    At what relative elevation was your fuel? lol 2.2ft/psi
     
  8. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    Stands are calibrated at absolute pressure, so atmospheric pressure is already taken into account.
     
  9. Vern

    Vern Full Access Member

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    so you only need to put your fuel on the roof to get test bench accuracy!
     
  10. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    No you need a properly working lift pump or an electric pump with a regulator
     
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  11. Vern

    Vern Full Access Member

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    Yes.

    Have you noticed changes in timing running pumps without supply assistance on the bench up to 400cc?
     
  12. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    When these pumps are calibrated on the stand they run 5 psi inlet pressure, if you run it without any supply pressure fuel quantity drops and timing retards.
     

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