Exhaust Gas Recirculation: Description and Operation
Exhaust Gas Recirculation:
SYSTEM OPERATION
The Exhaust Gas Recirculation (EGR) system is used to lower NOx (oxides of nitrogen) emission levels that are caused by high combustion temperatures. The EGR system introduces exhaust gas into the air/fuel mixture, to reduce combustion temperatures. The reduced combustion temperatures help to lower NOx emission levels. The main elements of the system are the EGR valve and EGR solenoid. The EGR valve is mounted on the intake manifold, and is vacuum operated. The solenoid is mounted on the engine valve cover and controls the amount of vacuum to the EGR valve.
Positive Backpressure EGR Valve:
EGR VALVE
This vehicle uses a positive backpressure EGR valve. A positive backpressure EGR valve requires exhaust backpressure (proportional to engine flow) to open and allow exhaust gas to flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, the engine might misfire. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. Careful diagnosis of the system is important so that outside conditions (such as an exhaust tube connected to the vehicle causing reduced exhaust backpressure) will not lead to misdiagnosis.
The EGR valve is usually open under the following conditions:
^ Warm engine operation.
^ Above idle speed but below wide open throttle.
EGR SOLENOID
The PCM controls the vacuum to the EGR valve with a solenoid valve. A constant 12 volts is applied to the positive terminal of the EGR solenoid. The vacuum supply to the EGR valve is regulated by the PCM controlling the ground to the EGR solenoid.
The percentage of time that the PCM grounds the EGR solenoid is called the solenoid duty cycle. The duty cycle is the time the solenoid is ON divided by the time it is OFF. A duty cycle of 0% will enable EGR (turn EGR full on) since a de-energized solenoid allows vacuum to pass to the EGR valve. A duty cycle of 100% will turn EGR full off since the solenoid will be energized and will not allow vacuum to pass to the EGR valve.
The EGR pulse width is regulated by the PCM depending on engine load conditions. When the engine is cold, within a specified load range and above a specified RPM, the solenoid valve sends 100% duty cycle to the solenoid and blocks vacuum to the EGR valve. When the engine is warm, the PCM sends a duty cycle to the solenoid to allow EGR operation.
The PCM uses the following sensors to control the EGR solenoid:
^ Coolant Temperature (CTS)
^ Throttle Position (TPS)
^ Manifold Pressure (MAP)
^ Throttle Switch (ISC)
^ RPM data from distributor reference pulses.
^ Vehicle Speed Sensor
During cold operation and at idle, the solenoid circuit is grounded by the PCM. This blocks vacuum to the EGR valve. Too much EGR flow tends to weaken combustion, causing the engine to run roughly or stop. With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may occur:
^ Engine stops after cold start.
^ Engine stops at idle after deceleration.
^ Car surges during cruise.
^ Rough idle.
If the EGR valve should stay open due to a stuck open valve all of the time, the engine may not run. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:
^ Spark knock (detonation).
^ Emission test failure.