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Greetings,

I've been chasing an odd fuel pressure issue with a 912UL.  In cruise, the pressure will initially be normal and steady, typically around 4.6 psi.  After 40 min or so, it will slowly drop to as low as 3psi, then climb back to nearly normal.  This pattern will continue for the rest of the flight. 

At this point I've ruled out everything but the mechanical pump, but before I replace it, I thought I'd ask if anyone has seen a mechanical pump do this?  It's the new style, less than 5 years old, with about 90 hours on it.

Thanks,

Rusty

 

10025_1_fuel pressure sag.jpg (You do not have access to download this file.)
  • Re: odd dips in fuel pressure, pump problem?

    by » 10 months ago


    Hi Rusty

    The pattern looks like vapor locking (vapor in the lines) 

    Consider what the temperature is and try some high octane fuel.  ( I would suggest some AVGAS since it is a predictable level of the RVP of the fuel) Just my opinion. 

    Cheers


  • Re: odd dips in fuel pressure, pump problem?

    by » 10 months ago


    Thank for the thought.  I only run 93 octane non-ethanol, and this has been going on for more than one fuel purchase. The only change since last summer is that I now have direct airflow to the pump, and more flow through the cowl than I ever did before.  I've monitored cowl temps as well, and they're much lower than last year.  That would make me not really suspect vapor lock. 

    I went 77 hours with no fuel pressure issues, then this started about 14 hrs ago.  The engine never misses a beat, or runs rough, and you'd only know it happened if you were looking at the pressure.  The boost pump stops it, which does still sound like vapor lock, but I just don't believe it.  Mainly I'm just curious if this is a known failure mode of mechanical pumps.  

    Rusty

     


  • Re: odd dips in fuel pressure, pump problem?

    by » 10 months ago


    Hi Rusty,

    Does your 912UL engine have a fuel return line ?

    I only ask because I have read, I think in this Forum, that the 80 hp engines often don't/didnt in the past,  have a fuel return.


  • Re: odd dips in fuel pressure, pump problem?

    by » 10 months ago


    I didn't initially have a return line, but added it after seeing some over pressure during ground operations.  Mine only returns to the gascolator though, not to the tank.  I made that change last summer, so nothing that would have caused this recent issue.  The engine has been really well behaved, so nothing has been changed.  That's the mystery.

    Rusty

     


  • Re: odd dips in fuel pressure, pump problem?

    by » 10 months ago


    Randoms thoughts -

    Returning to the gascolator - Could returning to such a small vessel be the problem in that fuel pump pulses may be magnified causing fluctuation in pressure readings?

    Seems to me that your return should go back to a tank, via a Rotax recommend a #35 Mikuni jet. The restriction jet will tend to even out/dampen fuel pump pressure pulses on the delivery side. The tank will completely remove any pressure pulse influence on the fuel pump supply side. 


    Thank you said by: Rotax Wizard

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