Theory Of Operation; Air Induction System; Fuel System; Fuel Pump - Mercury 25 EFI Service Manual

Fuel System Operation
Special Tools
CDS G3 Diagnostic Tool Kit

Theory of Operation

Air Induction System

The air induction system consists of an intake manifold with an intake runner to each cylinder for a smooth air flow to the
combustion chamber. Each runner has a single fuel injector controlled by the ECM. The fuel injector has several very small
holes in the fuel injector nozzle tip, to help atomize the fuel when injected into the runner as the cylinder valve opens.
A single throttle body/shutter with an attached Throttle Position Sensor (TPS), along with the Idle Air Control (IAC), manages
the amount of air entering the induction system. A Manifold Absolute Pressure (MAP) sensor, and a Manifold Air Temperature
(MAT) sensor, send manifold pressure and temperature information to the ECM to regulate the fuel injector pulse width, and
modulates the IAC duty cycle.

Fuel System

The fuel system consists of a fuel line connector, a fuel filter, a low‑pressure mechanical fuel pump, a high‑pressure electric
fuel pump, a vapor separator tank, a high pressure fuel filter, fuel injectors, a fuel cooler, and a fuel pressure regulator. The
low‑pressure mechanical fuel pump draws fuel from the fuel tank, through the fuel line connector, and fuel filter. The fuel pump
then delivers it to the high‑pressure fuel pump inside the vapor separator tank. High‑pressure fuel is circulated through a fuel
cooler, and a high‑pressure fuel filter prior to supplying fuel to the fuel rail. The fuel injectors spray fuel into the intake manifold
as the valve opens. Fuel not injected into the fuel rail, flows through pressure regulator, and returns to the vapor separator tank.

Fuel Pump

The fuel pump is a mechanically driven diaphragm pump which is driven off of the camshaft. The pump base insulates the fuel
pump from the heat of the cylinder head/valve cover. If the engine runs out of fuel, or has a restriction on the vacuum side of
the pump preventing adequate fuel flow, the pump will make an audible clicking noise.

Vapor Separator

The vapor separator maintains a liquid fuel supply for the high‑pressure fuel pump that is located inside the vapor separator
tank. Fuel delivered from the mechanical low‑pressure fuel pump is supplied to the top of the vapor separator, and is controlled
by the inlet needle/float assembly. Pressurized fuel from the high‑pressure fuel pump, circulates through the fuel cooler, passes
through the high‑pressure fuel filter, and into the fuel rail where the fuel injectors control the duration volume of fuel entering the
combustion chamber. Excess fuel flows through the pressure regulator back to the vapor separator tank.
Page 3A-2
Interfaces CDS G3 software to SmartCraft network.
NOTE: This kit includes download instructions and a license key.
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© 2016 Mercury Marine
8M0114141
21124
21110
90-8M0105568 eng MARCH 2016