GM Caprice 1990-1993 Repair Guide

Idle Air Control (IAC) Valve


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Idle Air Control (IAC) valve - Throttle Body Injection (TBI)


Engine idle speeds are controlled by the ECM through the IAC valve mounted on the throttle body. The ECM 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). The movement of the pintle controls the airflow around the throttle plate, which in turn, controls engine idle speed. IAC valve pintle position counts can be observed using a Scan tool. Zero counts correspond to a fully closed passage, while 140 counts or more corresponds to full flow.

Idle speed can be categorized in 2 ways: actual (controlled) idle speed and minimum idle speed. Controlled idle speed is obtained by the ECM positioning the IAC valve pintle. Resulting idle speed is determined by total air flow (IAC/passage + PCV + throttle valve + calibrated vacuum leaks). Controlled idle speed is specified at normal operating conditions, which consists of engine coolant at normal operating temperature, air conditioning compressor OFF , manual transmission in neutral or automatic transmission in D .

Minimum idle air 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 idle speed is controlled by the ECM through the IAC valve. No adjustment is required during routine maintenance. Tampering with the minimum idle speed adjustment may result in premature failure of the IAC valve.


  1. Disconnect the negative battery cable. Disconnect the IAC valve electrical connector.
  3. Remove the IAC valve by performing the following:
    1. On thread-mounted units, use a 1 1 / 4 in. (32mm) wrench.
    3. On flange-mounted units, remove the mounting screw assemblies.

  5. Remove the IAC valve gasket or O-ring and discard.

To install:
  1. Clean the mounting surfaces by performing the following:
    1. If servicing a thread-mounted valve, remove the old gasket material from the surface of the throttle body to ensure proper sealing of the new gasket.
    3. If servicing a flange-mounted valve, clean the IAC valve surfaces on the throttle body to assure proper seal of the new O-ring and contact of the IAC valve flange.

  3. If installing a new IAC valve, measure the distance between the tip of the IAC valve pintle and the mounting flange. If the distance is greater than 1.102 in. (28mm), use finger pressure to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve. If reinstalling the