Why is my marine fuel pump showing low pressure?

Low delivery pressure on a fuel pump is a symptom with a short list of causes, and the discipline is to work from the service tank toward the pump before touching any set point. First confirm the service tank actually contains fuel and that its quick-closing valve has not tripped shut, then verify every valve in the suction line is fully open. A choked suction filter or strainer is the single most common cause: check the differential pressure across the duplex filter, change over to the clean element, and note how quickly it fouled, because a filter that blocks repeatedly is telling you the tank or the fuel itself is dirty.

Next rule out air ingress. Entrained air collapses delivery pressure and makes the gauge needle hunt or flicker rather than sit steadily low, and you may see bubbles in any sight glass. The usual entry points are suction-side flange joints, pump gland packing, drain and vent valves left cracked open, and a low tank level letting the suction vortex. Nip up joints, renew suspect gaskets, and keep the tank level sensible.

Then look at the pressure-regulating and relief arrangement. A relief or spill valve stuck open, held off its seat by dirt, or set too low simply recirculates fuel back to suction; a spill line running unusually hot is a strong clue it is passing. Check fuel temperature and viscosity at the same time. Heavy fuel that is too cold is hard to draw and overloads the pump, while overheated fuel becomes so thin it slips through worn clearances; most systems aim for an injection viscosity in the region of 10 to 15 cSt, so verify the heater and viscometer are doing their job.

If the suction side and valves are sound, the fault is usually inside the pump: worn gears, screws or plungers on a positive-displacement pump, rising internal leakage as the fuel warms up, or a slipping or damaged coupling. A quick, decisive test is to change over to the standby pump; if pressure recovers, the pump is condemned, and if it does not, the system is at fault.

Only after all of this should you adjust the regulating valve, and then only against a second trusted gauge and the maker's stated normal pressure. Log what you changed and why. If you find yourself re-adjusting every few weeks, you are masking progressive wear, and the pump should be opened up at the next opportunity.