Fuel pressure mystery

The aircraft is still at the test flying stage (stay close to airfield, have not climbed above 4,000ft above terrain) so when the Dynon gives a low fuel pressure warning the boost pump goes on and stays on, as a precaution. Next flight will see what happens when boost pump turned off at good safe altitude.

Thoughts on the Vent tube...

The mechanical pumps are diaphragm pumps.
Each rotation of the Camshaft pushes the diaphragm Out, drawing fuel In. 
A spring controls the fuel pressure by pushing the diaphragm back In. Pushing Fuel Out.
The fuel pressure is a function of the spring force and the ambient pressure behind the diaphragm.
The pressure produced is "Gauge " pressure relative to ambient.
Its absolute value varies with altitude but is constant relative to ambient. (Gauge Pressure)

The back side of the diaphragm is open to the ambient atmosphere via the vent tube.
The Vent tube is a safety feature in case the fuel pump diaphragm were to rupture, dumping fuel.
As the diaphragm pumps, the vent tube continuously "Breaths", maintaining the ambient pressure behind the diaphragm.

If the end of the vent tube is in a Low-Pressure zone, especially if beyond the cowling, the fuel pump pressure will also vary.
If the vent tube is long and/or narrow, it will present a restriction to the "Breathing",  also affecting the Fuel pressure.

Try temporarily removing the Vent hose to see if the pressure loss is eliminated.

Thanks Bill,

"Try temporarily removing the Vent hose to see if the pressure loss is eliminated."

Have already started down this track. Replaced vent tune running from fuel pump, to lower right firewall location, with short tube & sample bottle just below pump location - will fly later today with hope that  problem solved.

Bill,

That’s a very interesting theory. If you lower the pressure on the spring side of the pump diaphragm, then the effective pumping pressure is reduced, thus lowering the output pressure.  If the vent tube is located in the airflow, or a low pressure area, that seems very plausible.  That would explain what is being observed, and suggests that airflow at speed (not “flying”) is the culprit.  I’ll be very interested in the results of Sean’s vent tube relocation.   

Reporting in after .9 hr test flight -

Before TO, checked fuel pressure sensor has a small (about 1.5mm) hole in casing, to atmosphere - appeared to be clear.

After leveling out

Turned boost pump off, fuel pressure dropped to below 2 psi  - fuel pressure then returned to 4.5 psi & stayed there. This suggests to me that the pressure sensor may be taking time to "catch" up with change in air pressure.

After landing

Noted fuel pressure at 3500 rpm, about 4.5 -5psi

After shut down

Checked sample bottle on end of carburettor air box drain - about 15ml fuel. Seem I have a flooding/overflowing float chamber - will investigate tomorrow.

Checked sample bottle end of pump drain - bit hard to see but might be empty  - will investigate tomorrow.