Evaporative Emissions System: Description and Operation

Fig. 5 Evaporative Emission Control System:

Fig. 6 EVAP System Operations Chart:

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 fuel filler cap. The system holds fuel vapor collected from the fuel tank in the charcoal canister where they are stored until the engine warms to 129° F and the BVSV opens and allows engine vacuum open a check valve in canister. When air is drawn through the canister it opens another check valve in canister that pulls air from the fuel tank, this in turn pulls air into the tank through the check valve in the fuel cap.