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SERVICE MANUAL NUMBER 23
Fuel Metering System
General Description
The function of the fuel metering system is to deliver the correct amount of fuel to the engine
under all operating conditions. The fuel is delivered to the engine by individual fuel injectors
mounted in the intake manifold near each cylinder.
Cool Fuel Systems
This system was used on later MerCruiser MEFI-1, MEFI-2 and MEFI-3 ECM engines. This
system has a fuel cooler and the electric fuel pump inside a box on the lower, port side of
the engine. The fuel regulator for this system is mounted on the fuel cooler.
Early versions of the MEFI-1 systems had a second regulator mounted in the fuel rail. This
regulator does not control system fuel pressure. It is used to dampen fuel system pulsation
only. Later versions of the MEFI-1 and the MEFI-3 have this regulator removed from the fuel
rail.
The 7.4L MPI (L29) engines use a GM intake manifold, plenum, injectors and fuel rail with
the MEFI-2 and MEFI-3 system. A second regulator is mounted on the fuel rail at the fly-
wheel end of the engine. This regulator has no function in the system. A plug is in the fuel
return port and a hose is connected to the vacuum port to drain fuel into the engine if the
diaphragm fails.
Modes of Operation
The ECM looks at voltages from several sensors to determine how much fuel to give the
engine. The fuel is delivered under one of several conditions, called modes. All the modes
are controlled by the ECM and are described below.
STARTING MODE
When the ignition switch is turned to the crank position, the ECM turns ON the fuel pump
relay and the fuel pump builds up pressure. The ECM then checks the Engine Coolant Tem-
perature (ECT) sensor and Throttle Position (TP) sensor and determines the proper air/fuel
ratio for starting. The ECM controls the amount of fuel delivered in the starting mode by
changing how long the injectors are turned ON and OFF. This is done by pulsing the injectors
for very short times.
CLEAR FLOOD MODE
If the engine floods, it can be cleared by opening the throttle half way (50%). (Open throttle
handle until resistance from secondary throttle [Multi-Port only] is felt.) The ECM discontin-
ues fuel injector pulsation as long as the throttle is between 50 to 75 % and the engine rpm
is below 300. If the throttle position becomes more than 75% or less than 50%, the ECM
returns to the starting mode.
RUN MODE
When the engine is started and rpm is above 300, the system operates in the run mode. The
ECM will calculate the desired air/fuel ratio based on these ECM inputs: rpm, Manifold Abso-
lute Pressure (MAP) sensor, Intake Air Temperature (IAT) sensor and Engine Coolant Tem-
perature (ECT) sensor. Higher engine load (from MAP) and colder engine temperature
(from ECT) requires more fuel, or a richer air/fuel ratio.
Index
90-861326--1 MARCH 1999
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MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-33
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