Fuel Pressure Regulator
The fuel pressure regulator, located inside the fuel supply module, controls the amount of fuel pressure required for the engine
to run efficiently. The fuel pressure regulator changes the pressurization of the fuel depending on the demands of the engine.
Air pressure in the intake manifold changes to a higher or lower pressure during RPM changes. These manifold air pressure
changes are linked to the fuel pressure regulator. The changes that take place at the fuel pressure regulator are managed by a
hose that is connected to the intake manifold. The air pressure change that is positive or negative on the fuel pressure regulator
diaphragm, increases or decreases the amount of fuel that is returned back into the FSM. The fuel pressure dumped by the fuel
pressure regulator passes through a fuel cooler. Passing the excessive fuel through the fuel cooler prevents the high‑pressure
and low‑pressure fuel pumps from overheating.
Fuel Lift Pump
The fuel lift pump located inside the FSM pulls fuel from the fuel tank. The fuel lift pump is activated with 12 volts through the
main power relay and the fuel pump control relay when there is no engine rotation, and the key is turned to the ON position for
approximately three seconds. The propulsion control module (PCM) does not have the logic to detect the fuel lift pump
operation. In the event of a fuel lift pump failure, no fault will be generated and the engine will not run.
The fuel lift pump has fuel passing through it constantly when the FSM is full of fuel. A fuel recirculating check valve inside the
FSM is conjoined with the fuel lift port passage. When fuel cannot be pulled into the FSM from the fuel tank (FSM is full), the
check valve opens and allows the cooled fuel inside the FSM to pass through the fuel lift pump, preventing the pump from
overheating. The FSM is a volume displacement module, which replaces the fuel inside the FSM automatically without the
installation of a float switch or needle and seat.
Fuel System
Page 7A-8 © 2018 Mercury Marine 90-8M0146617 eng JULY 2018
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