The Exhaust Gas Recirculation or EGR system is one of several vehicle emission control systems. It helps reduce the amount of the nitrogen oxides (NOx) in the exhaust gases. Nitrogen oxides are normally formed in the process of combustion in the engine cylinders. However, their formation increases dramatically at higher combustion temperatures (above 1600 °C or 2912 °F).
Higher combustion temperatures are also harmful to the engine. One of the effects caused by high combustion temperatures is a pre-ignition or detonation (pinging), when the air/fuel mixture ignites in the cylinders not from the spark, but from excessive heat. Because it happens at the wrong time, before the spark, the detonation adds strain on engine components.
The EGR system reduces the combustion temperature by diverting a small portion of the exhaust gases back into the intake manifold. Exhaust gases are no longer combustible. Diluting the intake air with exhaust gases makes the air/fuel charge less combustible.
The engine computer (PCM) opens or closes the EGR valve to control the flow within the EGR system. The EGR valve connects the exhaust manifold to the intake manifold. The EGR valve is normally closed. There is no EGR flow when the engine is cold, at idle, or during hard acceleration. The EGR flow is at its peak during steady cruising under moderate load.
The PCM periodically tests the EGR system along with other emission control systems. If the flow is more or less than expected, the PCM detects a fault and turns on the Check Engine light on the instrument panel.
There are different ways to monitor the EGR flow. Some cars use an EGR temperature sensor installed in the intake part of the EGR system. When the EGR valve opens, the temperature on the intake side rises from the hot exhaust gases.
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