Capacitor Start for a fuel Pump

Discussion in 'Automotive Electronics' started by aruffnit, Oct 12, 2016.

  1. aruffnit

    Thread Starter New Member

    Oct 12, 2016
    I have a 12v, 13a fuel pump that doesn't always start with the ignition key. I have checked the circuit, cleaned all the connections and grounds, dielectric greased all these connections but from time to time the fuel pump fails to start. Electrical system otherwise checks out fine.

    If I tap on the fuel pump it will start and run fine. I have replaced it a couple of times now, rewired it, and just wish to give it a higher voltage kick at start up. What size capacitor should I use, and how would it be wired in.
  2. bwilliams60

    Active Member

    Nov 18, 2012
    You fail to provide a lot of details. Make, model, year, mileage of car establishes type of pump. Some pumps are prone to this problem and there are solutions for them.
    Have you tried running battery voltage direct to the pump a number of times to see if it starts every time that way. It may be that you have a weak connection between battery and pump.
    Have you done any voltage drop tests on the positive and negative side of the pump? What were the results?
    Adding a capacitor isn't the solution. If it was, engineers would have already done so.
  3. oldgit

    New Member

    Jul 20, 2016
    Please tell us the voltage measured across the pump when you turn the igintion on.

    Be aware that many fuel pumps intentionally do NOT run until the engine is cranking, this is a safety measure to prevent fuel being pumped out of a broken car following a crash.

    The pump may also contain a pressure sensor - its already at pressure so not running - your tapping of the pump is enough to trigger the pressure switch (if the pressure is very close).

    If you have full battery voltage at the pump and it still fails to start - try depressurising the system before ignition on - let us know if it then starts.

    Sorry to have dodged the original question but you will not obtain a higher voltage by adding a cap. You may be able to reduce voltage drop at the pump during starting though. Use a cap well above the highest voltage - 25v is a preferred value that springs to mind. Remember that a huge cap will have a huge inrush current and you will need to control this to prevent damage to the relay or scr switching the pump. Also know that during high load (cranking) you need to protect against the cap being dischared into other loads (starter motor).

    Have fun.
  4. aruffnit

    Thread Starter New Member

    Oct 12, 2016
    Thanks for your reply. the car is a 1974 Porsche 914 4-cylinder, which is now running modern EFI fuel injection (Electromotive brand) and runs on 100 octane race fuel. The car is predominately used for club racing which always ads a ton of failure opportunities. It failed on me in the middle of a corner last weekend which was very dicey, the car was low on fuel at that time but i had no indication of fuel pump gravitation. fuel pressure gauge read 50 psi. Tapping on the fuel pump brought it all back to life. The fuel pump has less than 24 hours on it since new. The car has an OEM electrical system in place with the original alternator. The voltage gauge typically reads 13 volts or so indicating to me that there is sufficient voltage to operate the car and the charging system is working.

    The pump is a Bosch 044 model high capacity EFI fuel pump. It runs on a separate circuit, where power comes directly from the battery kill switch through a relay using 12-gauge wiring. The switch side of the relay comes from the ignition switch which is also fed from the kill switch. All this is fused with a 25 amp fuse. The ground is direct to the nearest chassis ground not a sheet metal screw. The battery kill switch is just a big switch on the positive side of the battery that totally shuts on ALL electricity. The battery wiring is all new with 1/0 copper cable.

    I have tested the pump out of the car with jumper wires to a 12 volt jumper box power source. Sometimes I had to tap it other times not.
  5. crutschow


    Mar 14, 2008
    Wouldn't the engine block be a better ground?
  6. aruffnit

    Thread Starter New Member

    Oct 12, 2016
    Engine is in the middle of the car, battery is in the front. Engine ground is a chassis only ground.
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Seems you already have your answer and just want one of us to tell you it is wrong and you can make the intermittent fuel pump work perfectly if you just add a 5 cent gizmo and solder a wire to the steering wheel.
  8. aruffnit

    Thread Starter New Member

    Oct 12, 2016
    I'll measure the voltage across the fuel pump and reply probably later in the week next week. Not available until then.

    The fuel pump cut off for engine not running or other condition is intentionally disabled--this is a race car now. I have wired in a new circuit just for the fuel pump. It is controlled by the ignition switch and a separate fuel pump switch.

    Lastly, fuel tank is in the front of the car along with the fuel pump. So fuel pump is very close to the battery and the battery kill switch. I'll have to recheck the fuel pump grounding also.

    Thanks go you all.
  9. debe

    Well-Known Member

    Sep 21, 2010
    If you have to tap one of those pumps to make it start, when you hooked it up strait to the battery then its faulty. Posibly sticky brushes, also a quick way to damage those pumps is to run them out of fuel. Also make sure there is a filter sock in the inlet of the pump if its an in tank pump. If its an external pump make sure the in tank filter is still on the pickup pipe & is not damaged. These pumps do not like small specs of rubbish as it will jam them as they are a roller type pump.