If an engine-mounted pump is unable to draw fuel from a day tank, it is most likely because of:
1. The day tank location and the associated piping layout require suction that is beyond the capacity of the engine-mounted fuel pump (the engine data sheet will show the max suction head for the pump, and also the max discharge pressure for the return side of the pump – these are not very strong pumps). You can relieve excessive restriction by minimizing or eliminating piping elements that are not absolutely necessary (foot valves inside the day tank suction line, excess elbows, etc.).
2. Air is getting into the fuel line and keeping the pump from creating suction. A sight glass in the piping may help determine if air is coming into the line. Tighten all connections that may be allowing air intrusion.
3. The engine’s inline fuel filters (if any) are clogged and not allowing flow to the engine.
4. The engine-mounted pump is in need of service or replacement. You can add a vacuum gauge on the piping between the engine and day tank to measure the pump’s performance.