I've got an 80-HP marine diesel engine (in a sailboat, naturally).
I've got a the Parker/Racor 75500MAX dual fuel filter system with an attached vacuum gauge to monitor resistance.
For the first few hours of operation (less than 2 hours) everything looked good: pressure was right around -3" Hg, as expected for a fuel system with 1/4" line coming from the tanks.
Sometime after this 2-hour period, the pressure started creeping up and eventually reached astronomical (-20" Hg) levels. The engine, starved for fuel started to falter.
I switched to the second filter on this system and the pressure was still astronomical. There was a slight drop, but not a drop down to -3".
My primary filters were essentially new. The bowl in the bottom was clean.
I replaced my secondary filters, bled and primed, and when the engine started up, the pressure leaped up to -10" Hg and floated up from there until the engine died again.
How can both filters show astronomical pressures like that?
Does this mean my fuel tanks are so filled with crud that little or no fuel is flowing?
Or could it be that my fuel tank vents have clogged? (Maybe a wasp's nest in a vent?)
Or is there some other explanation?
How would I diagnose this?