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Fuel system holding pressure 7A engine

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  • Fuel system holding pressure 7A engine

    Hello all.

    I've just tested the fuel system pressure on my 7A and found that running at idle; the rail pressure is 3.7-3.8 Bar (hard to see as gauge needle bounces) and with FPR vacuum removed it goes to 4.1Bar. This is all good and bang on what it should be. I then switched off to check holding pressure which should be 3.2bar after 10 minutes falling to 3bar after 20. In the time it took me to go to the bathroom for an average duration afternoon pee, the fuel holding pressure had dropped to 1.5bar and seemed stable at that.

    Where is my pressure going?

    There are no leaks in the engine bay - no smell of fuel getting out past an o-ring at all and nothing i can smell elsewhere in or around the car.

    I was about to send the injectors off for ultrasonic cleaning as they've done 7 years and nearly 50,000 miles, they are not really old but could that much pressure leak so quickly through an injector leak that isn't noticeable in any other aspect through bad running? Hopefully the cleaning coincidentally fixes the holding issue.

    The engine seems fine in every respect and the pump primes the fuel pressure and starting is quick and crisp both cold and hot, i am just curious as to what is happening!

  • #2
    The pressure regulator spring will be weak and allowing the pressure to drop quicker, not an issue if its running nicely I'd not be concerned
    Panthero Coupé quattro 20vt
    Indigo ABY coupé
    Imola B6 S4 Avant

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    • #3
      Sorry, I should have mentioned I fitted an adjustable FPR last year so that 'should' be alright though what I didn't get round to testing the hold pressure with the old FPR.... I'm not overly concerned as it does run well, I'm just curious really. What stops the pressure leaking back into the tank?

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      • #4
        There is a spring loaded diaphragm/valve in the pressure regulator that requires X amount of pressure to overcome, once the valve sees X pressure from the fuel pump, the valve lifts and begins passing fuel back to the tank. The MAP reference then applies pressure / vacuum to the valve to increase / decrease fuel pressure to maintain a constant pressure differential between the fuel rail and manifold.
        Panthero Coupé quattro 20vt
        Indigo ABY coupé
        Imola B6 S4 Avant

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        • #5

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