Firebird V6-191 3.1L (1990)
Idle Speed/Throttle Actuator - Electronic: Description and Operation
Idle Air Control (IAC) Valve
IAC Valve Air Flow Diagram
DESCRIPTION:
The purpose of the Idle Air Control (IAC) valve assembly is to control engine idle speed, while preventing stalls due to engine load.
The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. 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 a "Scan" tool test equipment, which plugs into the ALDL
connector.
During idle, the proper position of the IAC valve is calculated by the ECM, based on battery voltage, coolant temperature, engine load, and engine rpm.
If the rpm drops below specification and the throttle valve is closed, The ECM senses the near stall condition and calculates a new valve position to
prevent stalling.
If the IAC valve is disconnected and reconnected while the engine is running, the resulting idle rpm may be wrong and resetting the IAC valve will be
required.
After running the engine, the IAC valve will reset when the engine is turned OFF. When servicing the IAC valve, it should be disconnected or connected
with the engine OFF.
If the ECM is without battery power for any reason, the programmed position of the IAC valve pintle is lost, and replaced with a default value. This
causes the engine idle to be unstable for a period of approximately seven minutes. To return the IAC valve pintle to the correct position, refer to
MAINTENANCE PROCEDURES/IDLE LEARN PROCEDURE.
Different shaped pintles are used for different IAC valve applications. The model B115 throttle body has an IAC valve with a 12 mm dual taper pintle.
Be sure to use the correct part number, with the correct pintle shape and diameter, when replacement is required.
The IAC valve affects only the idle characteristics of the vehicle. If it is fully retracted, too much air will be allowed into the manifold and the idle speed
will be too high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too slow. If it is stuck part way open, the idle
may be rough, and will not respond to engine load changes. A Code 35 should be set if a controlled idle cannot be maintained.