The idle speed of the engine is controlled by the Idle Air Control (IAC) valve.
The valve changes the amount of air bypassing into the intake manifold in response to electric current controlled by the
ECM.
When the IAC valve is activated, the valve opens to maintain the proper idle speed.
(cont’d)
IDLE
ADJUSTING
SCREW
FAST
IDLE
THERMO
VALVE
IAC VALVE
TO PGM-FI MAIN
RELAY
YEL/BLK
BLU/RED
Idle Control System
System Description (cont’d)
1. After the engine starts, the IAC valve opens for a certain time. The amount of air is increased to raise the idle speed
about 150—300 rpm.
2. When the engine coolant temperature is low, the IAC valve is opened to obtain the proper fast idle speed. The
amount of bypassed air is thus controlled in relation to the engine coolant temperature.
IDLE SPEED
(rpm)
ENGINE
COOLANT TEMPERATURE
1. When the idle speed is out of specification and the Malfunction Indicator Lamp (MIL) does not blink code 14, check
the following items:
Adjust the idle speed (see page 11-102)
Air conditioning signal (see page 11 -90)
ALT FR signal (see page 11-92)
A/T gear position switch signal (see page 11-94)
Neutral switch signal (M/T) (see page 11-96)
Clutch switch signal (M/T) (see page 11-98)
Starter switch signal (see page 11-100)
Fast idle thermo valve (see page 11-101)
Hoses and connections
IAC valve and its mounting O-rings
2. If the above items are normal, substitute a known-good IAC valve and readjust the idle speed (see page 11-102).
If the idle speed still cannot be adjusted to specification (and the MIL does not blink code 14) after IAC valve
replacement, substitute a known-good ECM and recheck. If symptom goes away, replace the original ECM.