Evaporative Emissions System: Description and Operation

Fig. 166 Evaporative Emission Control System:

Fig. 167 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 it is stored until the engine warms to 129° F and the BVSV opens, allowing 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 fuel tank, this in turn pulls air into the tank through the check valve in the fuel cap.