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Fuel pressure

in 912iS Technical Questions 6 years ago
Hi All,

I wonder if you have any advise regarding fuel pressure indications. Our system has a fuel pressure gauge which is on the up stream side of the fine fuel filer after the fuel pumps. We have this arrangement to watch for filter clogging.
In our 'clean' filter situation we see a fuel pressure of around 56psi with one pump running. After some use the pressure rises and can exceed 65psi. We have set our warning pressure at 60psi and frequently get warnings with both pumps running and a clean filter.
On one occasion I have seen 90psi but did have some notable debris in the filter when cleaned. Other than that we see a fine particulate if we back flush the filter which probably equates to a volume of around 15mm cubed, is this normal?
What do you feel would be a good limit to use for warnings?

TIA

John

  • Re: Fuel pressure

    by » 6 years ago

    I have frequently seen the same issue. I worked hard to figure it out and bumped up the warning level as well. If your fuel system is like mine, there is a bypass valve if the pressure gets too high. So the worst case if it goes high is the fuel will head to the injectors unfiltered. Nothing should actually break.

    I have found the one common denominator for me is when I use Premium auto gas. When primarily on that fuel my fuel pressure will wander quite a bit. When I use Avgas that fluctuation goes away. I have no idea why, just providing my symptoms. My plan now is to use more of a blend of fuels. Likely more half and half. I do use Decalin when filling with AVgas to minimize the lead issue.
      
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  • Re: Fuel pressure

    by » 6 years ago

    Hi Jaguar56,

    Thank you for your reply, we only use Mogas in our system. Do you know what pressure the relief valve operates at?
    I'm in the UK and have not heard of Decalin, I will investigate.

    Kind regards


    John
      
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  • Re: Fuel pressure

    by » 6 years ago

    Check out this thread. https://www.rotax-owner.com/en/rotax-forum/is-technical-questions/6387-fuel-system-overpressure-912is

     

      
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  • Re: Fuel pressure

    by » 6 years ago

    What is your fuel pressure BEFORE engine start?
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    From The 912is Operators Manual...
    Fuel Pressure ...
    Max. 3.2bar (46.5psi)
    Min. 2.8 bar (40.5 psi)
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    Keep in mind that this pressure is measured in relation to the Air Box pressure which is always a partial vacuum. A conventional gauge will measure about 45 psi engine OFF and ~35psi at Idle.
    A differential gauge will measure 45psi in both states.
    60psi is way out the norm and 90 psi is the fuel pump's internal safety pressure limiters opening.

    If you have a filter bypass valve, it typically bypasses the filter at 1bar (15psi).
    Making 45+15 = 60psi the highest pressure you should ever see with a 100% blocked filter.
    50psi would be a reasonable alarm point.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    You have a blockage after the Fuel sensor.
    Either the Fuel Filter is being restricted or something after the filter is causing the problem.

    The primary/Coarse Filter before the fuel pumps filters to the 100u level.
    If there is any debris large enough to be visible in the secondary/fine filter, something is deteriorating!
    The fuel hoses could be shedding their linings and the hose after the second filter could be contaminating the engine.

    Remove the fuel line from the fuel Pumps and the Fuel rail on the engine.
    hold a paper towel over the far end to catch any debris.
    Blow through it. There should be minimal restriction and spotlessly clean fuel on the towel.
    The second filter will almost filter out bacteria. If you see Anything, it is way too Big!

    If nothing is found; remove the fuel return line from the fuel regulator back to the fuel tank
    Blow again. Open Hose?

    Then re-attach the fuel lines to the engine and remove the Banjo Bolt from the Fuel Regulator.
    Do not lose the two copper washers.
    Blow again, it should be an open free flowing hose and clean fuel only.

    All that remains is the Fuel Regulator.
    Remove the little 2-inch length of vacuum reference hose that goes from the regulator to the Airbox and verify that it is functional and not cracked. A leak will raise the fuel pressure.
    The only thing holding the Fuel regulator in its housing is a single internal retaining Ring.
    Lift out the Regulator and inspect for debris.

    Do Not ignore this problem!

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: RotaxOwner Admin
      
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  • Re: Fuel pressure

    by » 4 years ago

    Sorry, late updating this thread. This problem was eventually traced to the Fuel Pressure regulator. There is a mesh filter which was holding some fibres and also a slice of the sealing 'O' ring in the filter. All has been normal since this has been addressed.

     

    Regards,

     

    John

    Thank you said by: RotaxOwner Admin, Bill Hertzel
      
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