S10/T10 P/U 2WD V6-262 4.3L VIN Z TBI (1995)
Idle Air Control (IAC) Valve: Description and Operation
Idle Air Control (IAC) System
All engine idle speeds are controlled by the control module through the Idle Air Control (IAC) valve mounted on the throttle body. The control
module sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move "IN" or "OUT' a given distance (number
of steps) for each pulse (called counts).
This movement controls airflow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position
counts can be seen using a Tech 1 scan tool. Zero (0) counts correspond to a fully closed passage, while 140 counts or more (depending on the
application) corresponds to full flow.
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Actual or "controlled" idle speed is obtained by the control module positioning the IAC valve pintle. Resulting idle speed is generated from the
total idle air flow (IAC/passage + PCV + throttle valve + vacuum leaks).
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Controlled idle speed is always specified for normal operating conditions. Normal operating condition is engine coolant temperature in
operating range, the A/C is "OFF," manual transmission is in neutral or automatic transmission in drive with proper park/neutral position. A
high or low engine coolant temperature, or A/C clutch engagement may signal the control module to change the IAC counts.
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The minimum idle speed is set at the factory with a stop screw. 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 normal controlled idle operation. The IAC counts will
be higher than normal on an engine with less than 500 miles, or an engine operating at high altitude or an engine with an accessory load such as
the alternator, A/C, power steering or hydra-boost brakes activated.
