Holding Fuel Pressure

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

  1. J3bba

    J3bba He Who Struggles

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    When the ignition is turned to "On" and the fuel pump primes, it's meant to be holding the fuel pressure when the pump stops right?

    Mine isn't doing that and just checked the fuel filter/rail/regulator and none of them are leaking. What part is meant to hold the pressure after the pump primes?

    Edit: I've removed the fuel damper as well if that makes a difference.
     
  2. Shane001

    Shane001 Well-Known Member

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    Yes it should hold pressure. You've got a leak somewhere.
     
  3. J3bba

    J3bba He Who Struggles

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    Any usual places? It's not in the engine bay, checked everything there.
     
  4. Shane001

    Shane001 Well-Known Member

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    if you can't find it leaking anywhere it may be a faulty injector or faulty fpr.
    How quickly does it drop pressure?
     
  5. J3bba

    J3bba He Who Struggles

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    It drops pretty quickly. Injectors aren't leaking, I had the rail out and watched them hold pressure. I doubt it's the pressure reg, it gets up to the pressure it should then when the pump turns off the pressure disappears.
     
  6. Fists

    Fists Well-Known Member

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    Does it drop quickly after running or only after priming? If it's been sitting the prime duration isn't long enough to make sure all the vapour is cleared out of the lines.
     
  7. J3bba

    J3bba He Who Struggles

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    Engines out at the moment but it did it before I pulled it.

    When it ran it would hold pressure perfectly where it was meant to. It only drops during priming after the fuel pump has stopped. If I stop the car after running the fuel pressure drops back down again, more or less instantly. If the fuel pump is running I have pressure if it's not I don't.
     
  8. LitlElvis

    LitlElvis Z32 Servant

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    I don't believe the fuel system will hold pressure with the damper removed... I know mine doesn't.

    I view the damper as more of a check valve that helps to keep the rails pressurized for faster startup than a damper. Internally it's a carbon copy of the regulator with the only difference being the damper has a higher pressure setting than the regulator.
     
  9. Fists

    Fists Well-Known Member

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    Should hold without the damper, mine does, just one of the silver oem fprs and stock fuel pump. I would guess a check valve in the pump assembly has failed.
     
  10. Fists

    Fists Well-Known Member

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    Did some testing to double check my info, mine holds idle pressure, about 36psi, not full base pressure, drops from ~43 to 36 instantly once the pump stops but stays there for 10 minutes or so. Reinstalling the damper I had lying around didn't change it's behaviour. That's just from running the pump at full voltage, engine off.
     
  11. J3bba

    J3bba He Who Struggles

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    Having a look around I think the fuel check valve has stopped working. Didn't even realise the Z had one and I'm not really sure where it is.

    Thinking I might get a generic inline fuel check valve and see if that solves the issue.
     
  12. Fists

    Fists Well-Known Member

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    I think the check valve would have to either be built into the pump or the over-pressure relief valve does the job (basically another fpr mounted between the pump and bulkhead). Never paid much attention to the stuff in the tank though.
     
  13. Boorkus

    Boorkus Member

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    This has me curious as well...
    I have noticed something similar where the car holds fuel pressure fine with the pump running (43 PSI) but as soon as the pump turns off it drops 10-20PSI and slowly creeps back down to zero within 5-10 mins. Is it supposed to hold 43 PSI with the pump turned off, for significant time?
     
  14. Tektrader

    Tektrader Z32 Hoe, service me baby

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    Most stock Z32's will still be registering fuel pressure around 30 PSI even after sitting all night.
     
  15. beaver

    beaver southern zeds

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    some after market fpr's dont hold pressure against the diaphragm by design, the stock reg, as graham says, will hold pressure for quite a long time.
     
  16. SuperZ

    SuperZ Resident Z lunatic

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    I actually just posted with same problem and then saw this - I am having exact the same problem!

    Nothing to do with Pressure Loss at the rail and nothing to do with the Brand New Regulator or the Damper ....
    (My system passed all the respective tests) and the problem can only be from the regulator back (and it is definitely not the regulator !)

    So it can only be some sort of check valve at the fuel tank (as per what other mechanical sites are saying)

    Don't suppose anyone here knows where it is located etc ?

    Clearly there has to be one - because that is the only thing left.....
     
  17. Martin Williams

    Martin Williams Active Member

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    Any reason you would want it to hold pressure when ignition is turned off?
     
  18. East Coast Z

    East Coast Z Well-Known Member

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    Re: "Don't suppose anyone here knows where it is located etc ?"

    Using a 1993 2.0 Seater TT as the basis for a parts search.
    Part No. 17370-30P00 Check Valve Assembly is located at the fuel pump.
     
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  19. SuperZ

    SuperZ Resident Z lunatic

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    Thanks East Coast but the only check valve I can see - is on the pump side - which I know is good and holds pressure....
    (There doesn't appear to be one for the return line as such - nor does it make sense to have one - being a low pressure line) ....
    Which leaves only the fuel regulator again not holding pressure - if I clamp the fuel line return after the regulator - it holds pressure no problems but as soon as I release the clamp - all pressure is lost - which means it can only be at the regulator .... even though it maintains the 43.4 PSI when turning over

    The problem here is - the FPR 1200 turbosmart reg is brand new ....

    So I am now waiting for the manufacturer (Turbosmart) to get back to me with why the regulator is brand new and not holding constant pressure - (even after adjustment ) .... see what they say I guess ...
     
  20. Shane001

    Shane001 Well-Known Member

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    Probably just the design of the turbosmart fpr.

    Remember a fpr is not holding all fuel flow, it's allowing fuel to pass back to the tank depending on how much is needed at the engine. Pressure is not the same as flow. So once there is no more flow the pressure is released.
     
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