Fuel pressure drop on manual pump only

I was testing at 2500 rpm, with and without the electric pump running. When I turn off the electric pump the pressure drops over 15 seconds to “0”, then builds back up to 3.5. Is this an indication of a bad mechanical pump or ??

So I did a flight to see how it reacted under load at high RPM’s and more flow demand. In a climb at 5100 RPM I turned off the electric pump and the furl pressure dropped over a period of 10 seconds to 1.0 psi. Then it came up to 4.8 and again dropped to 1.1. This up and down continued for 3-4 cycles within one minute and finally stabilized at 4.3. When I turned the electric pump back on the pressure jumped to 5.5 and stayed there.

Had the same problem and it only got worse as time went on. Replaced the pump and all is well.

What the **** Walt! Per Van's don't turn off the electric pump while flying!

Just what you wanted, a smart *** answer! Lol..

Seriously, I just removed a perfectly good 9 year old mechanical pump off my plane and replaced with new. I plan to keep the pump as a spare, but I would be happy to send it to you for testing, if you will just send it back when ur done.. Just let me know..

The Facet electric pump is a solenoid pump that shuttles a spool back/forth. Check valves cause the fuel to travel only in one direction. To be honest, I don’t know how the Facet pump would affect fuel flow to the mechanical pump if it was not running. I suspect it would be hindrance, acting like a restriction in the fuel line, and cause lower pressure drop at the inlet to the mechanical pump to the point where the mechanical pump might starve for fuel.

I put the new higher pressure electric pump in about a month ago. The previous pump was working fine but…….
I just added a fuel pump switch and was “playing” with it when I noticed this issue. I suspect it has always been this way just that pulling the fuse isn’t something I would do to test the difference between the manual only and both running.
I think things are fine and what I am seeing is a delay in the Dynon pressure sensor maybe averaging pressure over time. There is about 1.5 gpm more flow when both pumps are running and that is the extra return fuel caused by the higher pressure.