Fuel Pressure Regulator

Bob,

The fuel pressure regulator on the 912is works in reference to manifold pressure, so ideally the fuel pressure sender would also reference manifold pressure.  However, many fuel pressure senders reference outside air pressure. This is a common reason for a difference in fuel pressure readings with the engine stopped and running. Some electronic flight displays have built in correction for this, but it must be set up correctly. There are several posts on this site that discuss this, and they are worth reading.  

Of course what you are seeing may be a caused by something different, but understanding how your system is set up is good place to start.  If you can tell us what fuel pressure sender you are using, and what instrument is displaying fuel pressure, we may be able to help you understand what is going on.  

Thanks, Jeff.  I just did as you suggested and searched the forum (should have tried that before posting ?).  And yes, that’s the answer.  The system uses a Kavlico differential sensor and feeds a Dynon HDX EFIS.  I didn’t appreciate what the “differential” meant, but now understand.  Thanks again. 

Bob,

I think most inquisitive 912is owners end up going through the “Differential Pressure” learning curve. For modern fuel injected engines it’s an important concept because the fuel pressure - in relation to manifold pressure - is one factor that determines the rate of fuel delivery. 

A differential pressure sender would have an air tube running from the sensor up to the Airbox on the engine.  This allows the sender to sample manifold pressure, and I’m not aware that Kavlico offers a sender with a connection for this air tube.  I suspect you may have a gauge pressure sender, and that gauge pressure reading is then converted to simulate a differential reading by the Dynon HDX. 

If you had a true differential sender, then there should be no difference in the pressure reading between the motor running and stopped. This, in my opinion is ideal.  In this case there is no conversion by the flight display, the sender is simply reading fuel pressure in relation to the same airbox pressure that the fuel pressure regulator is reacting to. Remember, the fuel pressure regulator is fully functioning before the engine is started, but in this condition the airbox pressure is equal to ambient pressure.  

This may be simply academic, but I bring it to your attention because if your Dynon is set up to convert the signal as I suspect, it may be possible to better calibrate the system so that the reading is more similar (and accurate) with the engine running or stopped.  This would start with measuring the actual pressure at the fuel rail with a calibrated gauge.  I don’t have any experience with the Dynon HDX, but others here do.  

I know it’s a differential sensor because Dynon put out a SB regarding possible leaks with the older sensor.  When I replaced it, I initially configured it for the non differential sensor.  Naturally, it didn’t read correctly.  Dynon correctly guessed I had not set it up for the differential sensor, which was not in the old sensor database.  Dynon sent me a new sensor file, which had the differential sensor listed, and once configured correctly, pressure readings were as expected. It doesn’t have an air pressure sensing tube, but has a third wire, which I BELIEVE is for the differential functionality?  Anyway that solved my issue.