Fuel Pressure Query - Losing pressure rapidly after shut down

Discussion in 'Z32 Technical' started by SuperZ, Jan 28, 2019.

  1. SuperZ

    SuperZ Resident Z lunatic

    Messages:
    1,973
    Likes Received:
    0
    Trophy Points:
    36
    Hi All

    I was under the impression that a Fuel Regulator doesn't let any fuel past the rising rate spring unless the desired pressure is achieved (stock TT Fuel Pressure is 43.4 PSI) .........but clearly.... I must be wrong

    When I turn my motor over - I get the desired 43.4 PSI (with my after market TurboSmart) but as soon as I switch it off - it returns to zero

    It is as if the Fuel Regulator doesn't stop the flow at all - but simply restricts the flow to maintain pressure while having some sort of flow bypass - but it is definitely not a problem with the regulator - as my Tomei Regulator does the same thing as well.

    I have tested my fuel rail and I have no pressure leaks ! I have tested the pump and the pump back flow valve and both work with no problems (they give and hold pressure well) - The only thing responsible for loss of pressure is from the Regulator to the Tank (and I know it is not the regulator...)

    My understanding is that if other 300zx fuel regulator's hold their pressure after being switched off - then there must be a drain back flow valve of some sort in the tank itself....... and mine must be failing..... as everything else passes the pressure test .... anyone know if this is correct... ?

    Or do you lose pressure as soon as you switch off as well?

    Cheers for any help
    Regards - Jamie
     
  2. Shane001

    Shane001 Well-Known Member

    Messages:
    2,552
    Likes Received:
    40
    Trophy Points:
    48
    Fuel pressure will drop off once you turn the pump off, but shouldn't drop to zero immediately.

    What gauge are you reading this off? Really silly question but if it's electric when you turn off the ignition the gauge will loose power and go straight to zero ;)

    Is it causing any issues when the car is running?
     
  3. kickerzx

    kickerzx Member

    Messages:
    231
    Likes Received:
    4
    Trophy Points:
    18
    There is a valve in the tank that usually is forgotten

    Try to have a buddy pinch the line before the filter when you shut it off. Does it hold pressure now? If it does you have your direction
     
  4. rob260

    rob260 Moderator Staff Member

    Messages:
    14,769
    Likes Received:
    171
    Trophy Points:
    63
    Location:
    Sydney (North)
    Hi Steve where is the valve? I didn’t think there were any valves on the bulkhead apart from the pressure release on the TTs?
     
  5. kickerzx

    kickerzx Member

    Messages:
    231
    Likes Received:
    4
    Trophy Points:
    18
    In the pump itself if im not mistaken. You probably have one in stock? Try blowing the wrong way see if it is possible.
    Anyways, the pinch test will tell you if thats the way the pressure leaks of
     
  6. kickerzx

    kickerzx Member

    Messages:
    231
    Likes Received:
    4
    Trophy Points:
    18
    Could be a loose clamp in the tank too but it would have to be pretty loose to drop pressure right away. Also it would struggle to actually build pressure in the first place
     
  7. kickerzx

    kickerzx Member

    Messages:
    231
    Likes Received:
    4
    Trophy Points:
    18
    And of course you can check the fpr the same way. Pinch the line at it and see if it hold pressure
     
  8. MagicMike

    MagicMike Mod and Event Coord [QLD] Staff Member

    Messages:
    9,183
    Likes Received:
    104
    Trophy Points:
    63
    I agree, I was sure the valve is built into the stock pump.

    I'm wondering why people are concerned about pressure drop after shut off? As long as the pump is priming to fire, I only want pressure while the engine is running.
     
    SuperZ and beaver like this.
  9. SuperZ

    SuperZ Resident Z lunatic

    Messages:
    1,973
    Likes Received:
    0
    Trophy Points:
    36
    I have spoken to the Engineer who designed the Turbosmart Fuel Pressure Regulator .... and here is what he told me...

    The Turbosmart FPR has an inbuilt bleed to allow the fuel pressure to adjust quicker to a surge of RPM/ Throttle - thus unlike some other regulators, the pressure is not constant when shut off - only when operating...

    Some regulators use constant pressure with no bleed ... when these shut off, they hold pressure constant but this does not apply to my regulator being an FPR AN6 (500+)

    The only difference between a regulator with constant pressure and a regulator with a bleed bypass - is that the car is already at pressure when starting the next day, whereas a fuel pressure regulator with a bleed will require the pump to prime it first (which happens every time you start the car) .... but either regulator ...should not stop the car from starting... just one will be pressurised from the start and the other will use the ignition pump prime for pressure before starting....

    Fuel Regulators must have the engine operating to set correctly, due to the pump receiving a higher voltage when operating (from 12V to 14V)

    Hope that helps...... (it helped me ....)

    P.S. for info:

    The check valve is built into the Pump - if you block off the fuel filter and apply a gauge there - the pressure should shoot upwards (excess of 100 psi) and then hold once clamped off (if your check valve works correctly)

    Clamping off before or after the fuel regulator - should also hold your pressure constant ..... if you don't have any leaks in your hoses, injectors, fuel rail etc.... with the regulator being the last device to test before the fuel returns to the tank via the low pressure line

    Cheers all
    Jamie
     
  10. SuperZ

    SuperZ Resident Z lunatic

    Messages:
    1,973
    Likes Received:
    0
    Trophy Points:
    36
  11. ewschinzel

    ewschinzel Member

    Messages:
    60
    Likes Received:
    0
    Trophy Points:
    6
    Location:
    Brisbane
    Maybe a leaking injector, doesn't take much to loose pressure.
    If the fuel does not drain back into the tank (faulty non return valve in the pump) and you have no leaking fuel hoses the only way the pressure can drop is through a leaky injector.
    Its also a good idea to clean the electrical terminals of the injectors..Z cars are fussy.
    A good injector cleaner might help a bit...
     

Share This Page