Holding Fuel Pressure

Discussion in 'Z32 Technical' started by J3bba, Dec 20, 2018.

  1. ivan129

    ivan129 Member

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    Hi, I haven't read the full thread but thought I'd add a couple of notes. The fuel rail should hold fuel pressure while the engine is running. Thats what the regs job is. I have a Sard rising rate reg set for 40PSI (thats what I see on the fuel pressure gauge when idling) and when under boost the reg lifts the fuel pressure 1 PSI for every 1 PSI boost. If I turn the engine off it holds the 40 PSI in the fuel rail because I'm using a one way valve in the fuel feed line. Some pumps have one way valves and others don't. I think my Walbro does but I also have an inline also. This stops fuel bleeding back through the fuel pump when the engine is off. I can leave the car for weeks and still see the fuel pressure gauge showing 30+ PSI. I'm guessing it does leak down a little and as temps come out of the cooling engine and sitting around. Hope that's of help.
     
  2. beaver

    beaver southern zeds

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    This may answer some questions..


    ) I’ve installed my new Aeromotive bypass regulator. Fuel pressure seems to adjust fine and holds great when the engine is running, but when I shut the engine off, pressure drops quickly to zero. Shouldn’t the pressure hold like it did with a stock regulator?

    No, Aeromotive EFI bypass regulators may not seal perfectly when the pump is off. They are engineered for the highest possible performance when the engine is running. OEM regulators must hold pressure for 30-minutes after shut-down to pass EPA emissions standards. At Aeromotive we know our customers priority is to have the best possible flow and pressure control when the engine is running and we don’t compromise this standard to force the regulator to seal when the engine is off. If extended crank or hard-start becomes a concern, first allow the pump to run and prime as long as possible after turning the key to the run position, then start cranking the engine, just before the pump shuts off. If the priming cycle is too short to allow the engine to start easily with this approach, extend the priming cycle in the ECU if programmable, or ad a timer board or momentary button to the fuel pump run circuit.

    7.) My regulator doesn’t seem to keep consistent pressure, every time I look at the gauge it reads something different, it’s driving me crazy, what is the problem?
    Chances are good you have a liquid filled fuel pressure gauge, infamous for changing its reading with temperature. Because a liquid filled gauge is sealed to keep the liquid inside, the pressure inside the gauge case may not be equal to atmospheric pressure. Once sealed shut, the liquid in the case expands and contracts as gauge temperature changes, making the internal pressure in the gauge vary up and down as it heats and cools. Case pressure can change as much as 7 psi up and down with heat, which affects the gauge mechanism and changes the gauge reading by the same amount!
     
  3. East Coast Z

    East Coast Z Well-Known Member

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    It's resistance to flow that creates pressure.
     
  4. ewschinzel

    ewschinzel Member

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    The check valve is built into the fuel pump.
    Could also be a loose hose clamp on the pump intake , obviously that is all inside the tank.
     
  5. ewschinzel

    ewschinzel Member

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    The fuel non return valve is built into the pump...there is a pulse dampener located on pump as well, silver colored membane pod.
     
  6. SuperZ

    SuperZ Resident Z lunatic

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    Just a follow up with my ... so called issue with fuel pressure losses after switched off ... I was used to the stock regulator holding pressure constant after switched off and after pressure testing everything (lines/ rails / valves etc) and clearing it all - except for the fuel regulator losing pressure quickly after switch off - I ended up going to the regulator manufacturers engineer, found out that the FPR1200 has and needs a bleed bypass to allow for acceleration surges with higher HP, so my problem ...is not actually a problem but part of the design. As long as the specified pressure is constant when cranking / idling - all is fine. Cheers
     
  7. J3bba

    J3bba He Who Struggles

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    So update. I've installed an inline check valve just before the fuel filter. It still loses pressure but it isn't immediate now, it'll quickly go down to zero but it isn't instant like before.

    I'm thinking it might be the fuel pressure reg? Are they designed to hold pressure once the fuel stops flowing? Is mine a bit busted then? I've got a turbo smart 800.
     
  8. SuperZ

    SuperZ Resident Z lunatic

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    Hi Jabba

    As stated earlier - I spoke to the Turbosmart engineer who designed the FPR
    (I have a FPR AN1200 myself and had the same problem)

    Unlike other FP Regulators that hold constant pressure (like the stock FPR for example) - the turbosmart actually has a bleed bypass feature and will lose pressure immediately once the pump stops flow (at switch off) - the purpose of the bleed bypass is to allow for more consistent surges on acceleration with cars running above 500 HP+

    As long as the regulator is holding constant pressure when the engine running - it is fine!

    See other thread (Fuel pressure) if you want the specs link for the bypass bleed performance vs other FPR's
    Hope that helps....

    Cheers.....
     
    jetett, Boorkus and J3bba like this.
  9. J3bba

    J3bba He Who Struggles

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    Perfect, thanks for the info.
     
  10. J3bba

    J3bba He Who Struggles

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