Low Fuel Pressure w/ Backup Pump On

Just had another look/read - the black cylinder is a fuel filter.

If this was my aircraft; I would delete the large opaque fuel filter and install one, in line, clear/see- through, on each tank supply line  (preferably before the pump).

My preferred filters are: Hengst  H 103 WK and H 102 WK

Almost indestructible. Easy to inspect. Gauze (rather than paper) filter. Light weight (carry a spare or two). So cheap, they are easier to replace than clean.

Thanks for the ideas, Sean.  In the past, I've added grounds directly to the instrument panel for the tach and the oil pressure gauge and it definitely helped in the case of the tach so I'll consider it for the fuel pressure gauge as well.  In fact, I may have already done it for the fuel pressure gauge when I did the others but I don't remember for sure so I'll check.  I agree with you that 2 filters, one at each wing, is safer and more reliable than the current setup in the event that the fuel filter becomes clogged so I'll look into that too.

Well, I have another thoughts related to the fuel filter setup. From the pictures it is clear that the both Facet pumps get unfiltered fuel.

Unfortunately, Facet requires the fuel to be filtered! They even recommend/require to instal a 74micron filter on a pump inlet for a reason.

https://www.facet-purolator.com/wp-content/uploads/2019/12/42664REVC_1-cube-E.pdf

I have the same Facet pump in my aircraft working already for ~950h without problems and when it is on, the fuel pressure always increases. But mine is behind a fuel filter.

Maybe your pumps are clogged with some debris and when being on just block the built-in valve while allowing even less fuel to flow than the valve itself when the pump in off?

How to check it? Buy a new pump. If it solves the problem buy also a second one and rebuild the fuel system to have two filters before the new pumps.

Good idea, Stefan.  When I add the wing tank filters, I'll place 'em just before (upstream of) the Facet pumps.  Thank you.

Dont know what happened to my comments above regarding ; Preferred fuel filters - they are  Hengst H 102 WK & H 103 WK

Almost indestructible. Easy to inspect. Gauze (rather than paper) filter. Light weight (carry a spare or two). So cheap, they are easier to replace than clean.