914 Fuel Pressure Sensor

Craig,

The 914 fuel pumps can produce over 16 psi, and the fuel pressure regulator is set up to provide fuel to the carburetors at approximately (airbox pressure + 3.6 psi). This can vary by a couple psi, the Rotax published limits are actually between 2.18 and 5.08 psi above airbox pressure, with 3.63 psi being the median.

If you consider that your 914 may be getting 39 hg of manifold pressure at takeoff power, that’s about 5 psi over the ambient pressure that your gauge is referencing at sea level.  In other words, you could expect your gauge pressure reading to be about 5 psi higher then it would be with a differential gauge referencing airbox pressure. If you add fuel pressure of 3.6 psi to the boost pressure of 5 psi, this justifies your previous gauge pressure reading of 9 psi on takeoff, whereas the 14 psi you are now seeing does not seem correct. I would think you should check your gauge readings against a calibrated gauge and figure out what has changed.   

Once you are confident everything is working properly… Typically the EFIS can be configured to take the signal from a standard fuel pressure sensor (gauge pressure) and apply a correction for manifold pressure so that your reading is correct.  Ideally you would alway get a reading of around 3-4 psi when corrected for manifold pressure. This is possible because the EFIS has a manifold pressure input and can use this data to calculate the correction real time. Your EFIS install manual will outline this.  

OK, I'll see how to config the Garmin G3X I have to do that.  I'm hoping the 14 psi reading was just a loose wire.

If that all doesn't work I guess I'll change the recommended differential pressure sensor.  A bit more work...

Thanks!

Looking at the G3X install manual I don’t see that the gauge to manifold pressure conversion is available for the 914.  Contrary to my original statement, that seems to be limited to the FADEC engines.  Section 24.2.2.2 indicates that you should use the UMA 1EU07D differential pressure sender for the 914.   If installed as a differential sender, this would have an air tube from the sender to the airbox to reference that pressure.