Isuzu C22NE Workshop Manual page 466

6E1-296 DRIVEABILITY AND EMISSIONS
IDLE AIR CONTROL (IAC) VALVE
The purpose of the idle air control (IAC) valve is to control
engine idle speed, while preventing stalls due to changes in
engine load. The IAC valve, mounted in the throttle body,
controls bypass air around the throttle plate. By moving the
conical valve (pintle) in (to decrease air flow) or out (to increase
air flow), a controlled amount of air can move around the
throttle plate. If the RPM is too low, the ECM will retract the IAC
pintle, resulting in more air moving past the throttle plate to
increase the RPM. If the RPM is too high, the ECM will extend
the IAC pintle, allowing less air to move past the throttle plate,
decreasing the RPM.
The IAC pintle valve moves in small steps called counts.
During idle, the proper position of the IAC pintle is calculated by
the ECM based on battery voltage, coolant temperature,
engine load, and engine RPM. If the RPM drops below a
specified value, and the throttle plate is closed, the ECM
senses a near-stall condition. The ECM will then calculate a
new IAC pintle valve position to prevent stalls.
If the IAC valve is disconnected and reconnected with the
engine running, the idle RPM will be wrong. In this case, the
IAC must be reset. The IAC resets when the key is cycled on
ON then OFF. When servicing the IAC, it should only be
disconnected or connected with the ignition OFF.
The position of the IAC pintle valve affects engine start-up and
the idle characteristics of the vehicle. When the ECM detects
the 175 rpm higher engine revolution than desires engine rpm,
this malfunction detects an error in the IAC control logic.
Diagnostic Trouble code 35 will be set.
RUN MODE (APPLICABLE TO CLOSED LOOP
SYSTEMS)
The run mode has the following two conditions:
• Open loop
• Closed loop
When the engine is first started, the system is in "open loop"
operation. In "Open Loop," the ECM ignores the signal from
the heated oxygen sensor (HO2S). It calculates the air/fuel
ratio based on inputs from the TP, ECT, and MAP sensors.
The system remains in "Open Loop" until the following
conditions are met:
• The HO2S has a varying voltage output showing that it is hot
enough to operate properly (this depends on temperature).
• The ECT has reached a specified temperature.
• A specific amount of time has elapsed since starting the
engine.
• Engine speed has been greater than a specified RPM since
start-up.
The specific values for the above conditions vary with different
engines and are stored in the programmable read only memory
(PROM). When these conditions are met, the system enters
"closed loop" operation. In "closed loop," the ECM calculates
the air/fuel ratio (injector on-time) based on the signal from the
HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.