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Have posted this question in a older thread (below) but decided to start new one. 

Low fuel pressure indication (below 36 PSI- Rotax 912is operators manual states that minimum acceptable pressure is 36 PSI). This indication only happens while in flight under 3500 RPM (pictures below for reference). Minimum pressure on the ground after engine start is 37.5 at 1470 RPM and only goes up from here. I have a feeling that this is related to fuel tank pressurization ? You're welcome to chime in. Thanks. Braga

https://www.rotax-owner.com/en/912is-technical-questions/9744-low-fuel-pressure-indication?start=0

10405_1_N6308 low fuel pressure indication.jpg (You do not have access to download this file.)
10405_1_N6308 low fuel pressure indication1.png (You do not have access to download this file.)
10405_1_N6308 run-up 1.png (You do not have access to download this file.)
10405_1_N6308 run-up 2.png (You do not have access to download this file.)
  • Re: 912is low fuel pressure (in-flight only) under 3500 RPM

    by » 5 weeks ago


    Caio,

    I believe your problem is different than the one in the previous post you referenced. Yours looks to be a more common issue. This is most likely happening because you have a gauge type fuel pressure sender that references ambient (outside) air pressure. This is a problem because the 912iS engine regulates its fuel pressure in reference to airbox pressure, not outside air pressure. When the engine is at low power, the manifold pressure is low (under vacuum). At idle the difference in airbox pressure and ambient pressure is about 6 PSI, which is the same as the error in your fuel pressure reading.  If you add 6 psi to the fuel pressure you are seeing at idle on the ground (37.5), that gives you the correct reading of 43.5.  This offset changes some at different altitudes, but the reason is the same.  As you advance the throttle airbox pressure gets much closer to ambient pressure and the fuel pressure error becomes very small. 

    It seems like just about every manufacturer has this problem when they first start using the injected engines. You can live with this error, knowing why it happens, or you can correct it.  Your G3X EFIS has a setting that will correct this error, not perfectly, but pretty close.  Since the EFIS gets airbox pressure information from the Rotax CAN bus, it can do the math to give you a corrected reading. It’s just a matter of setting up the EFIS to do this. That said, the best solution is using a differential type sender that has an air tube running from the sensor reference port to the airbox. That way the native signal from the sender is correct. When set up correctly, the oil pressure should always read about the same, typically around 44 PSI.  

    The differential sender most people use is the UMA N1EU70D.  This is a 0-70 PSI differential pressure sender.  If your aircraft is an SLSA, you would need authorization from the manufacturer to make this change.   

     


  • Re: 912is low fuel pressure (in-flight only) under 3500 RPM

    by » 5 weeks ago


    Jeff, thanks for the detailed reply. I concur with you on the issue/solution. I will inform the manufacturer and proceed accordingly. 


  • Re: 912is low fuel pressure (in-flight only) under 3500 RPM

    by » 5 weeks ago


    Caio,

    the differential pressure sensor that you asked me about in the older post, is the exact one that Jeff recommended!

    Jamaica Scott


    Thank you said by: Caio Braga

  • Re: 912is low fuel pressure (in-flight only) under 3500 RPM

    by » 5 weeks ago


    Caio, 

    I see you are in Texas and I am curious if you have a Colt aircraft.  I know they only recently started offering the 912iS engine.  Like I said, a lot of manufacturers got this wrong at first.  


  • Re: 912is low fuel pressure (in-flight only) under 3500 RPM

    by » 5 weeks ago


    Bingo! In fact this is the first Colt with 912is. Good that we know this since we have another Colt with 912is coming out of the assembly line. By the way, I am surprised the need for this part is not mentioned in the Rotax installation manual.


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