Fuel Pressure Regulator; Fuel Cooler; Throttle Body; Sensors - Mercury 25 EFI Service Manual

Fuel Pressure Regulator

The fuel pressure regulator is mounted on top of the vapor separator. It maintains a stable fuel pressure between the
high‑pressure fuel pump and the fuel injectors. The pressure regulator consists of a spring‑loaded diaphragm which actuates a
valve/seat assembly. Excess fuel pressure unseats the valve inside the fuel pressure regulator, returning fuel to the vapor
separator tank. The excess fuel is channeled below the fuel level in the vapor separator tank through an internal pipe to prevent
fuel vaporization. The spring side of the diaphragm is vented to the ambient atmosphere, allowing barometric conditions to act
on the diaphragm in addition to spring pressure.

Fuel Cooler

The fuel cooler works as a fuel heat exchanger. It circulates engine cooling water through a manifold to cool the high‑pressure
fuel supply to the fuel injectors. Lowering the fuel temperature by removing heat from the circulating high‑pressure fuel,
prevents the formation of fuel vapors, and reduces the probability of fuel vapor lock.

Throttle Body

The throttle body is connected to the intake manifold. It controls the amount of air that is allowed into the intake manifold. The
throttle body affects the Manifold Absolute Pressure (MAP). When the throttle body valve is closed (idle), the air pressure inside
the intake manifold is in a vacuum state. When the throttle body valve opens, the vacuum condition of the intake manifold drops
proportionally to the position of the throttle body valve. At WOT, the condition of the air inside the intake manifold continues to
be in a vacuum state, but is significantly less of a vacuum than when at an idle position. These changes of the vacuum inside
the intake manifold, affects the MAP sensor, allowing the ECM to change the fuel injection strategy, and the ignition timing.

Sensors

Function of Sensors

The sensors are some of the key components for controlling optimum fuel efficiency, horsepower, and emissions for internal
combustion engines. Each sensor is designed to monitor for a specific condition or identify a location. These condition and
location sensors supply vital information to the ECM. The sensors, in conjunction with the ECM, manage the fuel system, the
ignition system, and the warning system, based on the information received from these condition and location sensors.
90-8M0105568 eng MARCH 2016
a
b
c
a - Vent to ambient atmosphere
b - High‑pressure fuel from fuel cooler
c - Excess fuel to VST tank
9451
21109
21112
© 2016 Mercury Marine
Fuel System Operation
Page 3A-3