ENGINE CONTROL SYSTEM 3-21
BY-PASS AIR SCREW / PASSAGE
Since the throttle valve is almost fully closed when idling / troll-
ing, the main flow of air necessary to maintain idling / trolling
speed passes through the by-pass air passage.
The by-pass air screw controls the flow of air through the pas-
sage and provides a means of partially adjusting the total amount
of air necessary for idling / trolling.
NOTE:
See page 2-14 for the by-pass air screw adjustment procedure.
IAC VALVE / PASSAGE
The IAC valve is a linear solenoid plunger type mounted on the
intake manifold.
Its purpose is to control the amount of intake air flowing from
the IAC passage.
The IAC valve is driven by the duty cycle signal from the ECM.
IDLE AIR CONTROL SYSTEM
OUTLINE
The ECM controls the duty cycle signal of the IAC valve to regulate a portion of the intake air flow to the intake
manifold.
This system is used for the following purposes :
• To keep idling / trolling at specified speed.
• To improve driveability when decelerating.
(Dash-pot effect)
• To improve engine starting and warm-up performance.
(Fast-idle function)
The sensors / switch shown below monitor current engine condition and send signals to the ECM. Based on
these signals, the ECM determines the optimum duty cycle (repeating “ON” time rate within a cycle).
A repeating ON / OFF signal at a specified rate (250 times a second) is then sent to the IAC valve.
CKP sensor :
Informs ECM of engine speed.
ECM
Signal
IAC valve
MAP sensor :
Informs ECM of intake manifold pressure.
CTP switch :
Informs ECM of throttle position (open / closed).
Cylinder temp. sensor :
Informs ECM of cylinder temperature.
Cap
By-pass air screw
Throttle body
Cap
By-pass air screw
Throttle body
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