Idle Air Control (Iac) Valve; Run Mode; Starting Mode; Throttle Body Unit - Isuzu 1999 RODEO Workshop Manual

6E1–434 RODEO X22SE 2.2L ENGINE DRIVEABILITY AND EMISSION

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
PCM 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 PCM 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 PCM 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 PCM senses a near–stall condition. The PCM
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 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. If the
IAC pintle is fully open, too much air will be allowed into
the manifold. This results in high idle speed, along with
possible hard starting and a lean air/fuel ratio. DTC
P0507 may set. If the IAC pintle is stuck closed, too little
air will be allowed in the manifold. This results in a low idle
speed, along with possible hard starting and a rich air/fuel
ratio. DTC P0506 may set. If the IAC pintle is stuck
part–way open, the idle may be high or low and will not
respond to changes in the engine load.

Run Mode

The run mode has the following two conditions:
f Open Loop
f Closed Loop
When the engine is first started, the system is in "Open
Loop" operation. In "Open Loop," the PCM 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:
f The HO2S has a varying voltage output showing that
it is hot enough to operate properly (this depends on
temperature).
f The ECT has reached a specified temperature.
f A specific amount of time has elapsed since starting
the engine.
f 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 PCM 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.

Starting Mode

When the ignition is first turned ON, the PCM energizes
the fuel pump relay for two seconds to allow the fuel pump
to build up pressure. The PCM then checks the engine
coolant temperature (ECT) sensor and the throttle
position (TP) sensor to determine the proper air/fuel ratio
for starting.
The PCM controls the amount of fuel delivered in the
starting mode by adjusting how long the fuel injectors are
energized by pulsing the injectors for very short times.

Throttle Body Unit

The throttle body has a throttle plate to control the amount
of air delivered to the engine. The TP sensor and IAC
valve are also mounted on the throttle body.
Vacuum ports located behind the throttle plate provide the
vacuum signals needed by various components. Engine
coolant is directed through a coolant cavity in the throttle
body to warm the throttle valve and to prevent icing.
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014RX040