Skylark L4-138 2.3L DOHC VIN D MFI QUAD 4 (1989)
Idle Speed/Throttle Actuator - Electronic: Description and Operation
Idle Air Control (IAC) Valve Assembly
IAC Valve Air Flow Diagram
DESCRIPTION:
^ Engine idle speed is controlled by the ECM through the idle air control (IAC) valve mounted on the throttle body. The 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 commanded
location (steps away from the seated position) can be observed as a number of counts, displayed on a "Scan" tool. The pintle movement controls the air
flow around the throttle valve, which in turn, controls engine idle speed.
^ The "controlled" or desired speed for all engine operating conditions is programmed into the Mem-Cal of the ECM. The engine speeds are based on
coolant temperature, Park/Neutral switch status, vehicle speed, battery voltage, and A/C pressure (if equipped).
^ The ECM "learns" the proper IAC positions to achieve warm, stabilized idle speeds desired for the various operating conditions. This information is
stored in "Keep Alive" memories (information is retained after 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 minimum 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 ECM can result in incorrect idle control or
necessity to partially depress the accelerator when starting until the ECM relearns idle control.
^ Engine idle speed is a function of total air flow into the engine based on IAC valve pintle position plus throttle valve opening plus calibrated vacuum
loss through accessories.
^ The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air to flow by the throttle valve to cause the
IAC 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. Low internal friction, provisions
for slight production variations, and the various altitudes production engines may operate at, the calibrated air rate at the minimum throttle position is too
low to allow most engines to idle. The throttle stop screw is covered with a plug at the factory following adjustment.
NOTE: Do Not attempt to remove the plug and readjust the stop screw, see ADJUSTMENT PROCEDURES/THROTTLE VALVE MINIMUM
POSITION Misadjustment may result in damage to the IAC valve or throttle body, and may cause a Code 35 to set.
