Mercury MerCruiser MCM 454 Mag MPI Service Manual page 493

MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
ACCELERATION MODE
The ECM looks at rapid changes in Throttle Position (TP) and provides extra fuel by increas-
ing the injector pulse width.
FUEL CUTOFF MODE
No fuel is delivered by the injectors when the ignition is OFF, to prevent dieseling. Also, fuel
pulses are not delivered if the ECM receives no distributor reference pulses, which means
the engine is not running. The fuel cutoff mode is also enabled at high engine rpm, as an
overspeed protection for the engine. When cutoff is in effect due to high rpm, injection pulses
will resume after engine rpm drops slightly.
DECELERATION MODE
The Idle Air Control Valve (IAC) is similar to a carburetor dashpot. It provides additional air
when the throttle is rapidly moved to the idle position to prevent the engine from dying.
REV-LIMIT MODE
A fuel cutoff function is enabled at higher engine rpm. When the ECM senses that the engine
has exceeded its specified maximum rpm, no fuel is delivered by the injectors. After the rpm
drops below the specified maximum rpm, the ECM will resume fuel delivery.
MEFI-3 LOAD ANTICIPATION MODE
The Load Anticipation mode is available on MIE inboard engines only. The function is used
to help inboard engines during shifting. An electrical signal from the neutral safety switch
(on the transmission) is used to tell the ECM if the switch is closed or open. In neutral gear,
the neutral safety switch is closed (signal grounded). When shifting into gear, the switch
opens (signal open).
When the transmission is shifted into gear, the open signal causes the ECM to add a cali-
brated amount of bypass air with the IAC. This is done to increase the load handling capabili-
ty of the engine when going into gear on larger boats. When shifting back into neutral gear,
the additional IAC bypass air is removed in an attempt to limit engine rpm flares. The amount
of IAC air used is constantly monitored by the ECM. After the transmission is shifted, and
the engine has stabilized, the ECM calculates an 'error band' from the Moving Desired rpm
mode and adjusts the Load Anticipation mode IAC count accordingly. This allows the ECM
to 'learn' the best IAC bypass air position to use for shift conditions in each particular boat.
MOVING DESIRED RPM MODE
IMPORTANT: An improperly adjusted throttle cable can cause the engine idle rpm to
be higher than the normal 600 rpm even when the control lever returns to the idle rpm
position.
A Moving Desired RPM mode has been added to the MEFI 3 ECM. this mode will increase
the desired idle rpm to a calibrated set point according to the throttle position. When the
Throttle Position (TP) sensor is at the closed throttle setting, the ECM will use Idle Air Control
(IAC) and Ignition Control (IC) to maintain the calculated desired rpm. This will smooth the
transition from idle (closed throttle) to higher throttle settings. It will also help maintain
constant low engine speeds from approximately 600 to 1200 rpm. At 5% or greater TP sen-
sor setting, the Moving Desired RPM mode is not active.
Index
Page 5B-34
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SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999