First Engine Run Low Fuel Pressure

I have a similar situation in my 2016 SLSA airplane. Either pump alone will produce 43+ lbs with the engine off.  However with the engine at 2000 RPM (standard idle) either pump alone produces about 37 PSI, and both pumps together produce 43+ PSI.   But, once in flight with the engine under a load either pump alone will produce 41+ PSI.  Since engine speed has no effect on the electric fuel pump performance, I have always assumed this has to do with the mechanical fuel pressure regulator on the engine, and that less pressure is requested by the regulator at low power settings. But the manifold pressure drop in relation to outside air (as you describe) makes even more sense.  I will check my setting in the G3X.  

If you are using a gauge type sender and correcting the reading with the value from the MAP sender, there will be some inherent inaccuracies from both senders that will cause non-linear readings. 

The worst case is at engine idle and at sea level.

This will result in almost a 10 psi error if using an uncorrected "Gauge" reading sender.

The fact that you are only seeing a 1-2 psi shift indicates that the MAP is being taken into account.

For a proper reading, you should use a Differential Pressure sender.

They have two hose connections.

The WET connection connects to the Fuel Line.

The DRY Connection connects to the Air-Box.

UMA Differential Senders:

N1EUMA070D-A  (70 psi Differential, 1/8 Pipe)  (Aircraft Spruce)

N1EUMA100D-A  (100 psi Differential, 1/8 Pipe)

The N1EUMAxxx gauges are Non-TSO.

The T1EUMAxxx gauges are TSO.

- - -

An installation issue is that the Gauge and the sender MUST be connected to the same power source. (12v and GND).

This means that all three wires from the sender MUST run all the way back to the power terminals ON the gauge.

If the Sender gets it 12 volts in the engine compartment and the gauge is powered from the panel, the difference of even 0.1 volts will cause variations as the electrical loads vary.

Thanks Bill, that’s great information.  The manufacturer of the aircraft, BRM, used a Kavlico sensor that does not have the connection to the air box.  It is interesting that the preflight checklist they provide uses 36+ PSI as the minimum single pump fuel pressure for a 2000 RPM run up check.  It was not until I read the Rotax manual that I realized this was different than the 40+ PSI they want. 

Since the plane is an SLSA any changes would require an LOA, and the aircraft is functioning as per their spec.  So I think maybe the right thing to do is nothing, and proceed with the full understanding of why the pressure indications are what they are.  Plus, I can still compare each pump against the other and the minimum reference pressure of 36+ PSI at idle. I can also continue to verify the single pump pressure at cruise is 41+.  Thanks again to you and the original poster, it’s great to fully understand this issue.