Evaporation Control System

The evaporation control system (ECS) is used to prevent fuel vapors from entering the atmosphere after the engine is stopped. Vapors that are emitted from the engine and fuel tank when it is stopped are channeled to a canister. When the engine starts, they cannot be immediately purged (drawn into the engine) or the air/fuel mixture will be too rich. However, when the oxygen sensor is operating, it can compensate for the vapors.

The ECS system operates in conjunction with the air switching system. When air injection is shifted downstream, the vacuum signal which keeps the canister purge valve closed is shut off. Engine vacuum draws vapors out of the canister and into the engine, where they are burned. When the engine is stopped, vapors again flow from the engine and fuel tank to the canister. See Figure.