Daewoo NUBIRA, Owner'S Manual

Page: 229 / 744

ENGINE CONTROLS 1F – 407
DAEWOO V–121 BL4
A fault in the heated oxygen sensor (HO2S) heater ele-
ment or its ignition feed or ground will result in lower oxy-
gen sensor response. This may cause incorrect catalyst
monitor diagnostic results.
EXHAUST GAS RECIRCULATION
VALVE
The Exhaust Gas Recirculation (EGR) system is used on
engines equipped with an automatic transaxle to lower
NOx (oxides of nitrogen) emission levels caused by high
combustion temperature. The EGR valve is controlled by
the powertrain control module (PCM)/engine control mod-
ule (ECM). The EGR valve feeds small amounts of ex-
haust gas into the intake manifold to decrease combustion
temperature. The amount of exhaust gas recirculated is
controlled by variations in vacuum and exhaust back pres-
sure. If too much exhaust gas enters, combustion will not
take place. For this reason, very little exhaust gas is al-
lowed to pass through the valve, especially at idle.
The EGR valve is usually open under the following condi-
tions:
S
Warm engine operation.
S
Above idle speed.
Results of Incorrect Operation
Too much EGR flow tends to weaken combustion, causing
the engine to run roughly or to stop. With too much EGR
flow at idle, cruise, or cold operation, any of the following
conditions may occur:
S
The engine stops after a cold start.
S
The engine stops at idle after deceleration.
S
The vehicle surges during cruise.
S
Rough idle.
If the EGR valve stays open all the time, the engine may
not idle. Too little or no EGR flow allows combustion tem-
peratures to get too high during acceleration and load con-
ditions. This could cause the following conditions:
S
Spark knock (detonation)
S
Engine overheating
S
Emission test failure
INTAKE AIR TEMPERATURE
SENSOR
The Intake Air Temperature (IAT) sensor is a thermistor,
a resistor which changes value based on the temperature
of the air entering the engine. Low temperature produces
a high resistance (4,500 ohms at –40
³
F [–40
³
C]), while
high temperature causes a low resistance (70 ohms at
266
³
F [130
³
C]).
The powertrain control module (PCM)/engine control
module (ECM) provides 5 volts to the IAT sensor through
a resistor in the PCM/ECM and measures the change in
voltage to determine the IAT. The voltage will be high when
the manifold air is cold and low when the air is hot. The
PCM/ECM knows the intake IAT by measuring the volt-
age.
The IAT sensor is also used to control spark timing when
the manifold air is cold.
A failure in the IAT sensor circuit sets a diagnostic trouble
code P0112 or P0113.
IDLE AIR CONTROL VALVE
Notice : Do not attempt to remove the protective cap to
readjust the stop screw. Misadjustment may result in dam-
age to the Idle Air Control (IAC) valve or to the throttle
body.
The IAC valve is mounted on the throttle body where it
controls the engine idle speed under the command of the
powertrain control module (PCM)/engine control module
(ECM). The PCM/ECM sends voltage pulses to the IAC
valve motor windings, causing the IAC valve pintle tomove
in or out a given distance (a step or count) for each pulse.
The pintle movement controls the airflow around the
throttle valves which, in turn, control the engine idle speed.
The desired idle speeds for all engine operating conditions
are programmed into the calibration of the PCM/ECM.
These programmed engine speeds are based on the cool-
ant temperature, the park/neutral position switch status,
the vehicle speed, the battery voltage, and the A/C system
pressure (if equipped).
The PCM/ECM ”learns” the proper IAC valve positions to
achieve warm, stabilized idle speeds (rpm) desired for the
various conditions (park/neutral or drive, A/C on or off, if
equipped). This information is stored in PCM/ECM ”keep
alive” memories. Information is retained after the ignition
is turned OFF. All other IAC valve positioning is calculated
based on these memory values. As a result, engine varia-
tions due to wear and variations in the minimum throttle
valve position (within limits) do not affect engine idle
speeds. This system provides correct idle control under all
conditions. This also means that disconnecting power to
the PCM/ECM can result in incorrect idle control or the
necessity to partially press the accelerator when starting
until the PCM/ECM relearns idle control.
Engine idle speed is a function of total airflow into the en-
gine based on the IAC valve pintle position, the throttle
valve opening, and the calibrated vacuum loss through ac-
cessories. The minimum throttle valve position is set at the
factory with a stop screw. This setting allows enough air-
flow by the throttle valve to cause the IAC valve pintle to
be positioned a calibrated number of steps (counts) from
the seat during ”controlled” idle operation. The minimum
throttle valve position setting on this engine should not be
considered the ”minimum idle speed,” as on other fuel in-
jected engines. The throttle stop screw is covered with a
plug at the factory following adjustment.
If the IAC valve is suspected as the cause of improper idle
speed, refer to ”Idle Air Control System Check” in this sec-
tion.