Lanos S Hatchback L4-1.6L DOHC D-TEC MFI (1999)
Idle Air Control (IAC) Valve: Description and Operation
NOTE: Do not attempt to remove the protective cap and readjust the stop screw. Misadjustment may result in damage to the idle air control (IAC) valve
or to the throttle body.
The idle air control (IAC) valve is mounted or the throttle body where it controls the engine idle speed under the command of the powertrain control
module (PCM)/engine control module (ECM). The PCM/ECM sends voltage pulses to the IAC valve motor windings, causing the IAC valve pintle to
move in or out a given distance (a step or count) for each pulse. The pintle movement controls the airflow around the throttle valves, which in turn,
controls the engine idle speed.
The desired idle speeds for all engine operating conditions are programmed into the calibration of the PCM/ ECM. These programmed engine speeds are
based on the coolant temperature, the park/neutral switch status, the vehicle speed, the battery voltage, and the A/C system pressure (if equipped).
The PCM/ECM "learns" the proper IAC valve positions to achieve warm, stabilized idle speeds rpm) desired for the various conditions (park/neutral or
drive, A/C on or off, if equipped). This information is stored in PCM/ECM "keep alive" memories (information is retained after the ignition is turned
OFF). All other IAC valve positioning is calculated based on these memory values. As a result, engine variations due to wear and variations in the
minimum throttle valve position (within limits) do not affect engine idle speeds. This system provides correct idle control under all conditions. This also
means that disconnecting power to the PCM/ECM can result in incorrect idle control or the necessity to partially depress the accelerator when starting
until the PCM/ECM relearns idle control.
Engine idle speed is a function of total airflow into the engine based on the IAC valve pintle position, the throttle valve opening and the calibrated
vacuum loss through accessories. The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air flow by the
throttle valve to cause the A/C valve pintle to be positioned a calibrated number of steps (counts) from the seat during "controlled" idle operation. The
minimum throttle valve position setting on this engine should not be considered the "minimum idle speed," as on other fuel injected engines. The throttle
stop screw is covered with a plug at the factory following adjustment.
If the A/C valve is suspected as being the cause of improper idle speed, refer to "Idle Air Control System Check.
