Idle Control System

Fig. 10 Idle control system schematic:

The idle control system, Fig. 10, controls engine idle speed via an electronic air control valve (EACV) and a fast idle valve. The EACV valve changes the amount of air bypassing into the intake manifold in response to electrical current sent from the ECU. When the EACV is activated, the valve opens to maintain proper idle speed
After the engine is started, the EACV opens for a predetermined length of time, increasing the amount of air to raise idle speed approximately 15--250 RPM.
When coolant temperature is low, the EACV is opened to obtain proper fast idle speed. The amount of bypassed air is thus controlled in relation to coolant temperature. When coolant temperature is below 122°F, it also activates the fast idle valve to prevent idle speed from dropping.

Fig. 8 Typical idle bypass circuit:

Because fuel cutoff occurs at a predetermined position or angle of the throttle valve and signals from the throttle position sensor are used to determine the amount of fuel injected into the engine, idle speed cannot be maintained by altering the position of the throttle plate. If the throttle valve is moved to adjust idle speed incorrect fuel delivery may result, adversely affecting driveability and exhaust emissions. To compensate for this condition, idle speed is maintained by allowing a calibrated amount of air to bypass the throttle plate.
Basic idle speed is set by the adjustment of the idle bypass adjuster screw, Fig. 8. This screw controls airflow through the throttle body bypass channel, thereby controlling the amount of air bypassing into the intake manifold without changing position of throttle valve. However, when the electronic idle control system is in operation, the idle adjusting screw has no effect on idle speed.
During normal operation idle speed is controlled by the amount of air allowed to bypass the closed throttle plate through various solenoid valves such as the idle control, fast idle, A/T idle (if equipped), and A/C idle control solenoid valves. These valves are operated by the Electronic Control Unit (ECU) and, with the exception of the A/C idle control solenoid, they regulate the amount of air bypassed into intake manifold. The A/C control solenoid valve opens the throttle when the air conditioner is turned on, either by signals sent from the ECU, or by signals transmitted by the A/C system controller.