The exhaust gas recirculation (EGR) system is used to reduce the amount of
nitrogen oxide (NOx) emission levels caused by combustion temperatures exceeding
816°C (1,500°F). It does this by introducing small amounts of exhaust gas
back into the combustion chamber. The exhaust gas absorbs a portion of the thermal
energy produced by the combustion process and thus decreases combustion temperature.
The EGR system will only operate under specific temperature, barometric pressure
and engine load conditions in order to prevent drivability concerns and to increase
engine performance.
The EGR system consists of the following components:
| | The EGR valve is vacuum operated. The EGR valve is used to transmit exhaust
gases from the exhaust system to the intake manifold to be recirculated into the
combustion process. |
| | Vacuum for the EGR vacuum control system is created by a belt driven mechanical
pump called a vacuum pump. When the engine is running, the vacuum pump is operating
at all times. |
| • | EGR Valve Vacuum Control Solenoid |
| | The EGR valve vacuum control solenoid is located in the EGR vacuum control system
between the vacuum pump and the EGR vacuum vent solenoid. The ECM pulse width modulates
(PWM) the ground path of the EGR valve vacuum control solenoid allowing metered vacuum
from the vacuum pump to open the EGR valve to the desired position. The EGR valve
vacuum control solenoid is supplied ignition voltage through the ignition 1
voltage circuit from the Fuel HT fuse. The EGR valve vacuum control solenoid is normally
closed. |
| • | EGR Vacuum Vent Solenoid |
| | The EGR vacuum vent solenoid is located between the EGR valve vacuum control
solenoid and the EGR vacuum sensor. The ECM opens the EGR vacuum vent solenoid, allowing
vacuum from the vacuum pump to build in the EGR vacuum control system. When the
EGR vacuum vent solenoid is commanded closed, the EGR vacuum control system is vented
to atmosphere which will cause the EGR valve to close very fast. The ECM controls
the EGR vacuum vent solenoid by switching the ground path ON and OFF. Ignition voltage
is supplied through the ignition 1 voltage circuit from the Fuel HT fuse.
The EGR vacuum vent solenoid is normally closed. |
| • | EGR Throttle Valve Vacuum Control Solenoid |
| | The EGR throttle valve vacuum control solenoid is located in the EGR vacuum
control system between the vacuum pump and the EGR throttle valve. The ECM opens
the EGR throttle valve vacuum control solenoid, allowing vacuum from the vacuum pump
to close the EGR throttle valve. Diesel engines do not create enough engine vacuum
on their own to allow the EGR gases into the combustion process. The EGR throttle
valve, when closed, creates a restriction of incoming fresh air to the engine in
order to create engine vacuum. When the ECM commands the EGR valve to open, the EGR
throttle valve will be commanded closed. The ECM controls the EGR throttle valve
vacuum control solenoid by switching the ground path ON and OFF. Ignition voltage
is supplied through the ignition 1 voltage circuit from the Fuel HT fuse.
The EGR throttle valve vacuum control solenoid is normally closed. |
| | The EGR vacuum sensor is located in the EGR vacuum control system between the
EGR vacuum vent solenoid and the EGR valve. The ECM uses the EGR vacuum sensor to
monitor the amount of vacuum that is available to the EGR valve. The ECM will make
adjustments to the EGR vacuum control system in order to obtain the vacuum level
necessary to achieve the proper EGR valve position. The EGR vacuum sensor is interfaced
with the ECM by a 5-volt reference circuit, a low reference circuit and a signal
circuit. |
| | The MAF sensor is located in the air intake system between the air filter and
the EGR valve out-take port. The ECM monitors the Mass Air Flow (MAF) sensor signal
to calculate the actual amount of EGR flow into the intake manifold. When the EGR
valve is opened, the MAF rate will decrease. When the EGR valve vacuum control solenoid
is operated at 50-70 percent duty cycle, the MAF rate should drop at
least 4 g/s. |
| | The EGR valve cooler is mounted on the right intake manifold, between the EGR
valve and the exhaust pipe. Engine coolant flows through the EGR valve cooler
in order to lower the exhaust gas temperatures before it enters the EGR valve and
intake manifold. |
| | The PCM calculates the amount of EGR needed based on the following inputs: |
| • | The accelerator pedal position (APP) sensor |
| • | The barometric pressure (BARO) sensor |
| • | The engine coolant temperature (ECT) sensor |
| • | The exhaust gas recirculation (EGR) vacuum sensor |
| • | The intake air temperature (IAT) sensor |
| • | The mass airflow (MAF) sensor |
| • | The vehicle speed sensor (VSS) |
