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I’m in a foreign country and I have a problem. Looking for som advice. Rotax 912is with about 100 hours. I have Garmin G3X and on a flight yesterday I started getting a low fuel pressure indication. Normally 43 psi but I noticed it was in the yellow arc at about 38 psi. By the time I landed 15 minutes later it was at about 36 psi. At the airport we removed and opened the course gas filter and it was clean like new. No debris. I dump the tanks before flights and it’s always clear. No water or debris. I’m running 93 octane mogas. 
I made another flight back to my temporary base. That flight was 1.3 hours. By the time I got there, fuel pressure was 29 psi. Bottom of the range on the indicator. Engine ran fine. On ground run up, I can turn off either the main or auxiliary fuel pump and engine runs fine. I can see a slight pressure drop when either pump is cycled off. 
what do you think?

Fuel Pressure sensor?

Rotax Fine Filter?

Pressure regulator?

bad pump assembly?

please respond asap!

  • Re: Low Fuel Pressure Indication

    by » 3 months ago


    1. Where is the fuel pressure sender located, before or after the fine filter?
    2. Before this started, was your fuel pressure fairly steady at 43 PSI during all flight and ground operations?
    3. Are you flying in unusually warm conditions?
    4. Does the pressure improve as you descend, or change with altitude? 
    5. You say you are in a foreign country, is this a different fuel source than you usually use?

    At 29 PSI I believe you would experience at least a partial power loss, which makes me think this is a fuel pressure sender problem.  However, fuel pressure is critical on the iS engine and I would not fly it until this is sorted out.  

     

     


  • Re: Low Fuel Pressure Indication

    by » 3 months ago


    I had similar issues a couple of weeks ago, but not as severe.  Couldn’t reach 5000’ without a low fuel pressure alarm.  Running 93 winter blend.  Flew yesterday with 50% AVGAS and haven’t seen better pressure readings.  Not sure if it’s available where you are, but you might try this before more invasive processes.   Or if you have wing tanks, MOGAS in one and AVGAS in the other for testing. 


  • Re: Low Fuel Pressure Indication

    by » 3 months ago


    From the photo, I believe that the fuel pressure sensor is before the fine filter. 
    I’ve been here in Jamaica flying for two months. 
    using 93 octane Mogas. 
    I agree. I don’t want to fly until we can get this resolved. 
    trying to get a technician with a mechanical pressure gauge to come to my airport. 


  • Re: Low Fuel Pressure Indication

    by » 3 months ago


    As Bob stated above, this could be a fuel vaporization problem since you are running Mogas and you don’t know what the vapor pressure of the fuel is there.  Typically you see this as you climb, and it does not show up at low altitudes.  With the injected engine, this is more likely if the fuel pump is higher than the fuel tank (low wing aircraft), and the fuel tends to vaporize on the suction side of the pump.  The solution is Avgas, which has a consistently low vapor pressure.  If the pressure sender is before the fine filter, then the filter is not the problem.  

    If you test the system with a mechanical gauge…

    Remember that when you use a simple mechanical gauge for testing you will see different readings with different throttle settings.  This is because the fuel pressure regulator operates in reference to airbox (manifold) pressure, whereas your gauge will be referencing ambient pressure. Using a differential type gauge with a separate air tube connection to the airbox would solve this, but you may not find one there.  

    Assuming you will use a standard mechanical gauge, at idle your pressure reading will read lower, probably around 36 PSI, and will rise to the low 40s as you increase throttle and manifold pressure nears ambient pressure. To correct the reading for a particular throttle setting, you need to add the difference in ambient and manifold pressure (both in PSI) to your gauge pressure reading.  So as an example, on a standard day at 29.92 hg that’s 14.7 PSI ambient, and if your manifold pressure is reading 26 hg, that’s 12.8 PSI in the airbox.  So the difference is 2.1 PSI, which you would add to your gauge pressure reading to correct it.  Mechanics not familiar with Rotax injected engines may not know this.  


     


  • Re: Low Fuel Pressure Indication

    by » 3 months ago


    Jeff,

    Thanks for your insight and detailed instructions on testing pressure. 
    On Sunday when this occurred, I remained under 2,700 feet in elevation for the flight where I discovered the issue and the 1.3 hour flight after we inspected the course filter. It was hot that day but about 67F at altitude. 

    If the problem is vaporization, then on a cold start the fuel pressure should return to normal, correct?

    Strange but good thing is that the engine seems to be running normally. Is their some safeguard built into the system that allows the engine to continue to run even though there may be a fuel vaporization problem?

    I have been running Mogas here for the last two months without a problem. This issue seemed to come on all at once, and continued to indicate lower PSI as the day went on. 

     


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