The exhaust gas recirculation (EGR) system controls the formation of oxides of nitrogen (NOx) emissions by recirculating the exhaust gases into the combustion chamber. NOx emissions increase with combustion chamber temperatures. Controlling the high combustion chamber temperatures will help limit the formation of NOx emissions. The EGR system accomplishes this by admitting controlled amounts of exhaust gas into the intake manifold to mix with the incoming air. Mixing exhaust gases with incoming air/fuel mixture modifies the combustion process resulting in lower combustion chamber temperatures.
EGR Valve
The EGR valve (1) is located on the left corner of the front engine bank. The valve uses a stepper motor to drive a worm gear. The worm gear is in mesh with a plunger that controls the exhaust gas passage opening. Plunger movement is monitored by a position sensor. The powertrain control module (PCM) uses a pulse width modulated (PWM) signal to operate the stepper motor. By rotating the stepper motor in different directions, the PCM controls the opening of the exhaust gas passage to the intake manifold.
Operation
The PCM uses the inputs from various sensors in order to control the operation of the EGR valve. Information from the engine coolant temperature (ECT) sensor, the throttle position (TP) sensor, the vehicle speed sensor (VSS), and the manifold absolute pressure (MAP) sensor are critical for proper EGR valve operation. Under heavy engine loads, when the demand for power is high, the EGR valve is closed. In order to insure a smooth idle and stable engine operation, the EGR valve is also closed at closed throttle engine operation. The EGR valve is usually open during low and medium engine loads and speeds. Combining pre-programmed engine calibrations with various sensor inputs, the PCM calculates the optimum opening of the EGR valve.
The EGR valve position sensor enables the PCM to know if the valve plunger is actually in the desired position. Information from the sensor is used to increase or decrease the plunger opening, in order to achieve the correct flow. If the position sensor signal indicates that the plunger position is incorrect and cannot be adjusted, a diagnostic trouble code (DTC) will set.
Results Of Incorrect Operation of the EGR System
Excessive EGR valve flow may cause any of the following conditions:
| • | The engine stalling |
| • | Rough idle, surging, or engine hesitation |
| • | Lower fuel economy |
| • | Incomplete combustion and high exhaust emissions |
Inadequate EGR valve flow may cause any of the following conditions:
| • | Engine detonation |
| • | Excessive exhaust emissions |
