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Lycoming engine driven wobble pump, fuel pressure fluctuating

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  • Lycoming engine driven wobble pump, fuel pressure fluctuating

    I have had two engine driven fuel pumps on my engine in the last 700hrs. Both wobble pumps. The first one failed in the mechanism (not the normal gasket).

    Normally the fuel pressure is steady, but I noticed that both old and new pump occasionally fluctuate between about 23 (or 25) and 27 PSI. If I apply the electric boost pump, then I see the pressure rise to about 28 or 29 PSI and the fluctuation reduces, but there is still a small fluctuation. It can keep doing this for hours. Other times, the pressure will be steady for hours on end, I can't say why.

    Can anyone comment on possible causes? Should I be concerned?
    I thought I saw a thread about this issue somewhere recently, but I can't find it.

    Fuel pressure is measured at the outlet of the engine driven pump.
    Fuel pump is a certificated part / installation.

  • #2
    With my Lycoming 390 the fuel pressure is sometimes low for 1-2 minutes after warm restart. Never in flight nor any noticeable fluctuations. Any updates on your situation?


    • #3
      All we can figure is the transducer is modern enough to detect the pressure fluctuations which the pumps always cause. However I cannot substantiate that, and we would expect someone else with modern avionics and a wobble pump to mention the same.


      • #4
        Ya, I haven't found them to be the most reliable thing either over the years. Pressure fluctuates a lot considering, you can't count on more than a couple hundred hours out of them. It's like most expensive aviation things on our airplanes that hasn't changed since the 50's. Bring 2, because the reality is they're insanely unreliable... like mags and vacuum pumps


        • Battson
          Battson commented
          Editing a comment
          That is an interesting comment - all the aircraft engineers I have spoken to here in NZ say they've never seen on fail. That said, there are good shops and bad shops. I suppose the bad shops see more failures than the good.

        • zkelley2
          zkelley2 commented
          Editing a comment
          Battson, I think I've had one fail on every piston driven airplane with a pump I've ever flown. That includes my own airplanes, and 3 types for hire over 3 different air carriers.
          Never been an issue due to the electric backup, which is why it's there, but I cannot in good faith call those things reliable.

          Same with magnetos actually. The only piston airplane I've not had a mag failure on was my pacer, and that's probably because I only had it for ~110 hours.

          And the same with vacuum pumps in airplanes that were IFR. I'd probably die today if I had to shoot a partial panel fixed card NDB.

          Basically if it's attached to the accessory case, it's probably going to fail.

          This might be because most of my flying time is for hire, and so these things are flown a lot and they're not out there replacing things before overhaul time like a lot of people in GA do since the airplane flies so little in comparison and things just get old before they come anywhere near TBO.
          Also could just be pure exposure. I used to fly more per month than most GA pilots do in a year. More in a year and a half than most accumulate in a lifetime. You're just going to see a lot more.
          Last edited by zkelley2; 01-08-2021, 07:01 PM.

        • Battson
          Battson commented
          Editing a comment
          Excellent feedback Z - thank you.