Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation Valve Description

The Exhaust Gas Recirculation (EGR) system is used on engines equipped with an automatic transaxle to lower NOx (Nitrogen Oxide) emission levels caused by high combustion temperature. The EGR valve is controlled by the Engine Control Module (ECM). The EGR valve feeds small amounts of exhaust gas into the intake manifold to decrease combustion temperature. The amount of exhaust gas recirculated is controlled by variations in vacuum and exhaust back pressure. If too much exhaust gas enters, combustion will not take place. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. The EGR valve is usually open under the following conditions:
^ Warm engine operation
^ 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:
^ The engine stops after a cold start.
^ The engine stops at idle after deceleration.
^ The vehicle surges during cruise.
^ Rough idle

If the EGR valve stays open all the time, the engine may not idle. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause the following conditions:
^ Spark knock (detonation)
^ Engine overheating
^ Emission test failure