Evaporative Emissions System: Description and Operation

Fig. 10 Evaporative Emission Control System:

Fig. 11 EVAP Operations Chart:

This system is designed to prevent raw fuel vapors from escaping into the atmosphere by the use of a charcoal canister, vacuum switching valve (VSV), purge port at the intake manifold and a vented fuel filler cap. In addition to the fuel tank being vented, the carburetor float bowl is also vented to the charcoal canister by means of an outer vent control valve (OVCV). The system holds fuel vapor collected from the fuel tank and the carburetor in the charcoal canister where it is stored until the engine warms to 131° F and the temperature switch signals the ECU to open the VSV. Engine vacuum then draws the gas vapors into the carburetor through the purge port where they are spent in the combustion process. When air is drawn through the canister it opens a canister check valve that pulls fresh air through the system, this in turn pulls air into the fuel tank through the check valve in the fuel cap.