Evaporative Emissions System: Description and Operation

Fig. 6 Evaporative Emission Control System:

Fig. 7 Evaporative Emission Control System Operation:

This system is designed to prevent raw fuel vapors from escaping into the atmosphere by the use of a charcoal canister, bi-metal vacuum switching valve (BVSV), purge port at the intake manifold and a vented fuel filler cap. The system holds fuel vapor collected from the fuel tank in the charcoal canister where it is stored until the engine warms to 129° F and the BVSV opens and allows engine vacuum to open a check valve in the canister. When air is drawn through the canister, it opens another check valve in the canister that pulls fresh air through the system. This in turn pulls air into the fuel tank through the check valve in the fuel cap.