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My RV12iS is ready for the very first flight. I have concerns about the following fact:

Before engine start, fuel pressure with one elec pump 3 bar, with two elec pumps 3.1 bar. After engine start: fuel pressure with two pumps 2.8 bar, with one pump 2.7 bar and the „low fuel pressure“ warning comes on.
Engine running smooth.
Then, when increasing rpm above about 3000rpm, fuel pressure comes back to normal, 3 bar.
Whats wrong here? If i reduce the rpm again to 2200, pressure with 2 pumps again 2.8 bar.

Any idea, is this a problem or not?

Is this an issue to install the fuel pressure damper?

  • Re: RV12iS, Fuel Pressure low after engine start?

    by » 3 months ago


    What is your Bus voltage before and after start?

    The fuel pumps strain when the voltage drops much below 13v.

    How long are the wires from the Engine harness to the Fuel Pump switches and what gauge wire was used?

    16 awg is the minimum recommended for short runs. (5-6 ft)

    You might want to measure the actual voltage at the two pin connectors on the pumps.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: RV12iS, Fuel Pressure low after engine start?

    by » 3 months ago


    Bus voltage before engine start 13.1 V, with engine running 14V, so this is not an issue!?


  • Re: RV12iS, Fuel Pressure low after engine start?

    by » 3 months ago


    I get way more pressure than that before starting - around 46psi with both pumps and about 13.1 volts.  44 and change with one pump. If I were you I’d check first that there isn’t a major blockage somewhere, if not, check the pressure sensor.


  • Re: RV12iS, Fuel Pressure low after engine start?

    by » 3 months ago


    With the 912is engine the fuel pressure regulator operates with respect to manifold pressure (MAP).  So your fuel pressure gauge needs to either reference the air box pressure or be corrected for this on your display.  When your engine is off with the pumps running, manifold pressure equals outside air pressure so the gauge is accurate.  

    When the engine is running at idle, the MAP is much lower than outside pressure so the gauge is going to read low. When you increase rpm the MAP increases and the gauge reads closer to what the regulator is asking for based on MAP.  At WOT your MAP is once again very close to outside air pressure so the gauge is very close to correct.  

    There are fuel pressure senders that allow you to connect the reference source to the air box, which will then give the correct reading. Also, some electronic displays will have a setting that lets you use a standard sensor and correct for MAP.


    Thank you said by: Bill Hertzel

  • Re: RV12iS, Fuel Pressure low after engine start?

    by » 3 months ago


    Thank you very much, that makes sence!
    I measured 156 liter / h fuel flow, so, the pumps and the fuel lines are ok.


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