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Help needed on figuring out the issues with the fuel system

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  • Help needed on figuring out the issues with the fuel system


    Starting the car is taking longer than normal, I have done below tests but I am having difficulty in summing it up to a conclusion:

    - Connected a fuel pressure gauge to the inlet fuel line

    Question 1: Does the system supposed to hold pressure when not running

    - removed the fuel relay and jumped terminals 30 to 87, fuel pressure is 3 bars, however, the moment i stop the jump, it fells down to zero in 20 seconds. So the system is not holding pressure. Should it hold pressure? Is there a check valve at the fuel pump end of the system? To make sure that it is not the return line from the FPR is leaking, I clamped it at 3 bar and removed the jumper, same result, pressure dropping in 20 secs.

    Question 2: Should the fuel pump start running when the ignition is on (car not running)

    - Fuel relay removed, Ignition off, 30 and 87 jumped on fuel pump relay, fuel pump runs and continues running as long as jumped. Why fuel relay terminals are hot if no ignition? Is it a problem with the main relay? How can I test the main relay?
    - Fuel relay installed, ignition on (car not running), noting happens, fuel pump does not work. I was expecting the fuel pump to work for a couple of secs to build pressure before start.
    - Fuel relay installed, ignition start, car runs

    Question 3: Is the fuel pressure normal under different tests?

    - Idle with the FPR vacuum hose attached, 3 bars
    - Idle with the FPR vacuum hose dis-attached, very slightly above 3 bars. Isn't it supposed to go up more without vacuum?
    - Steady rev at 3000 RPM, 3 bars. I suppose this is ok
    - hit and release the gas pedal, a very slight increase followed by a decline to 2.7 bars and then 3 bars. I do not know if this is ok. What should the max reading be under full open throttle, still 3 bars?



  • #2
    1/ yes
    2/ no
    3/ yes
    75 M2 Zinno
    88 M3 Henna


    • #3
      Thank you for the answer, what do you think the reason would be for the system not holding the pressure? Is there a check valve in the fuel pump which might have failed?


      • #4
        Things that would bleed off fuel pressure:
        Leaking injector(s)
        Leaking Fuel Pressure Regulator
        Leak in any fuel line past the high pressure pump.
        jimmy p.
        87 E30 M3 Prodrive British Touring Car
        88 E30 M3 Zinnoberot - Street
        88 E30 M3 Lachsilber - Race (#98 SCCA SPU)
        92 E30 M Technic Cabrio - S14 POWERED!
        98 318Ti M44, Base - Morea Green
        04 Ford F350 - V10


        • #5
          I have not checked item 1 with a gauge but suspect it would drop to 0 at some point and it might not be the problem anyway.

          Just trying to confirm fuel problem or not.
          With the fuel gauge connected can you still replicate the taking longer than normal to fire up,
          during this time do you have the 3 bar fuel pressure and how long quickly does it arrive.

          I had a similar long cranking time with my M3 and the 5 year old fuel pressure relay was at faulty, once I installed the 25 year old one it was good again.

          E30 M3 1987
          Mini Clubman GT
          BMW E36 323 Msport
          Toyota Corona
          KTM 200EXC
          Honda CB50 (1979)


          • #6
            Hi Dave, actually I did put on my 32 years old FPRegulator today instead of the 3 year old, it gave 3.3 instead of 3 bars, starting was much more quick but it was leaking to the vacuum hose so I put back the new one.

            To your question, it takes 2 seconds to get to 3 bars.


            • #7
              Have you tried jumping the relay so the pump is running prior to attempting to start?

              If it starts better it will confirm it is the pressure loss causing the slow start.
              Sport Evo No.47

              My Sport Evo Restoration


              • #8
                Yes I did that and it did not start early, but if I start the car lets say 5 seconds after I shut down, it starts quicker. At the end of the day the conclusion I came to is the following:

                - there is no check valve at the fuel system of these cars so it is normal that the fuel will flow back to the tank when the car is not running. FPR has nothing to do with this. Installing a check valve is an option but why do that if BMW did not.
                - fuel pump is not running before engine start, hence no pressure is pre-built. Again this is by design.
                - the fuel pump only starts working when the DME sends the ground signal looking at the crank and cylinder position, so I think some cranking is needed anyway for the DME to get the position and give the signal to the pump, then the pump builds pressure. Lets say DME gives the ground in two seconds and the pump builds up pressure in another 2, 4 seconds of cranking is needed, which is almost my cranking
                - I think driving modern cars is making me question things that I should not question


                • #9
                  If you jumped the fuel pump relay and it made no noticeable difference in fire up time
                  I think it is safe to say it is not fuel delivery problem, even though by age the pump/s are probably due for replacement.

                  Probably best to visit the ignition system refresh that and take it from there.

                  E30 M3 1987
                  Mini Clubman GT
                  BMW E36 323 Msport
                  Toyota Corona
                  KTM 200EXC
                  Honda CB50 (1979)