Mercury MerCruiser MCM 454 Mag MPI Service Manual page 497

MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
IDLE AIR CONTROL (IAC) VALVE
The purpose of the IAC valve assembly is to control engine idle speed, while preventing
stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls
bypass air around the throttle valves.
By moving a conical valve known as a pintle, IN, toward the seat (to decrease air flow), or
OUT, away from the seat (to increase air flow), a controlled amount of air moves around the
throttle valve. If rpm is too low, more air is bypassed around the throttle valve to increase
it. If rpm is too high, less air is bypassed around the throttle valve to decrease it.
The ECM moves the IAC valve in small steps, called counts. These can be measured by
scan tool test equipment, which plugs into the DLC connector.
During idle, the proper position of the IAC valve is engine load, and engine rpm. If the rpm
drops below specification and the throttle valve is closed, the ECM senses a near stall condi-
tion and calculates a new valve position to prevent stalling.
Engine idle speed is a function of total air flow into the engine based on IAC valve pintle
position.
"Controlled" idle speed is programmed into the ECM, which determines the correct IAC
valve pintle position to maintain the desired idle speed for all engine operating condi-
tions and loads.
The minimum idle air rate is set at the factory with stop screws. This setting allows
enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a
calibrated number of steps (counts) from the seat during "controlled" idle operation.
If the IAC valve is disconnected and reconnected with the engine running, the idle speed
may be wrong. In this case, the IAC valve can be reset by doing the following: Turn off
engine, wait ten seconds, and restart engine.
IAC Valve Air Flow Diagram
Index
Page 5B-38
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SERVICE MANUAL NUMBER 23
72800
72986
90-861326--1 MARCH 1999