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fuel pressure a little too high

in 912iS Technical Questions 8 years ago
Hi everyone,
rotax 912is operators manual write:
fuel pressure:
min 2.8bar (40.5psi)
Max 3.2bar(46.5psi)

I install fuel system follow 912is manual, when we run engine, turn on one pump,the fuel pressure shows between max to min. but trun on two pump, the fuel pressure reach 46.8PSI In lower rpm, Increase the speed, The pressure became to normal value.

The mean is when trun on two pump in lower RPM, the fuel pressure has a little too high,Do you know Is this normal? Thanks.

  • Re: fuel pressure a little too high

    by » 8 years ago

    Nothing to get too worried about.

    First. Did you calibrate the Fuel Sender with the display? Each one close but slightly different.
    Remove the sender and temporarily replace with a reliable mechanical gauge. Do they agree?

    Is the sender AFTER the Fuel Pumps but BEFORE the Fine Fuel Filter?
    What size is the Filter?
    There are ~30 Liters per Hour of fuel flowing through the Filter with the engine running or NOT running.
    The filter needs to be fairly large to handle the flow.
    A small compact filter may be restricting the flow and raising the pressure.
    The typical filter is 50 mm or more in diameter.

    The Sender is reading Gauge Pressure.
    The 912iS Fuel Pressure RegulatorThe Regulator maintains a Differential Pressure.
    Look closely on the top of the back side of the regulator and you will notice a small (6mm) short hose running to the Air Box.
    The regulator maintains the fuel pressure relative to the Air Box Pressure (MAP).
    This pressure varies with Throttle and Altitude.

    To display the Actual, Differential Pressure you will need to correct for the Air Box Pressure, MAP Pressure, sent from the ECUs.
    Your Display may allow you to program the differential pressure.
    Otherwise, expect to the pressure to vary with throttle changes.

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

      
    Report

  • Re: fuel pressure a little too high

    by » 8 years ago

    Thanks
    but manual write "The limits for fuel pressure must not be exceeded"
    fuelpressure.jpg (You do not have access to download this file.)
      
    Report

  • Re: fuel pressure a little too high

    by » 8 years ago

    I see many instruments that are off anywhere from .5-2 psi all the time. Without actual pressure confirmation it may not be a real problem.

    Something to check.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: fuel pressure a little too high

    by » 8 years ago

    Stephen,

    Like Rodger said, your gauge may not be correct.
    We can not give you any recommendations without knowing what you have.

    Are you using the Rotax 912iS fuel pump assembly?
    What filters are you using?
    Where are you measuring the pressure?
    How are you measuring the pressure?
    Have you confirmed the pressure with a second gauging method? <---<<< #1
    When you turn on the second pump, how much does the pressure change? 1psi? 3 psi?
    When operated one at a time Do both pumps measure the same?

    So many questions! :)

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

      
    Report

  • Re: fuel pressure a little too high

    by » 8 years ago

    Thanks
    I use rotax pump and filter

    Follow the rotax schematic diagram,and use dynon sensor.


    Trun on one pump, the pressure about 44.5 PSI, Trun on two pump will reach 46.8PSI in lower rpm

    When we didn't run engine, just trun on two pump,the fuel pressure normal.
    pump.jpg (You do not have access to download this file.)
    sensor.jpg (You do not have access to download this file.)
      
    Report
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