Evaporative Emissions System: Description and Operation

Fig. 2 Evaporative emission control system schematic:

Fig. 3 Vapor storage canister cross sectional view:

This system prevents the emission of fuel vapors from the vehicle fuel system. The major components of the system are the vapor storage canister, switch vent solenoid and liquid vapor separator, Fig. 2.
Fuel vapors from the fuel tank and carburetor float chamber are vented to the canister which contains charcoal. When the engine is running, the fuel vapor stored in the canister is drawn into the intake manifold along with fresh air. The purge valve opens and allows the fuel vapors to be drawn into the intake manifold through the purge line. Air is drawn in through a filter in the bottom of the canister, Fig. 3.
Fuel vapors are not purged while coolant temperature is low. The BVSV bleeds off the vacuum which would otherwise be applied to the purge valve. As coolant temperature rises, the BVSV closes off the vacuum bleed, thus directing full manifold vacuum to the purge valve.

Fig. 4 Check valve inspection:

TESTING

TWO-WAY CHECK VALVE
Take care not to inhale fuel vapor inside valve.
1. Remove check valve with vacuum hose.
2. Using a suitable hand vacuum pump gauge, check valve as shown in Fig. 4.
3. Replace valve if found defective. Install check valve with orange side directed toward thermo valve.

Fig. 3 Vapor storage canister cross sectional view:

VAPOR STORAGE CANISTER
Take care not to inhale fuel vapor inside canister.
1. Disconnect battery ground cable.
2. Disconnect hoses from canister, then remove canister.
3. Hold pipes C and D closed with fingers, Fig. 3, then blow air into pipe A. Air should come out from pipe B, if not, replace canister.
4. Blow air into pipe B, air should not pass through pipes A, C or D, if not, replace canister
5. Blow air into pipe C, air should come out from pipes A, B and D, if not, replace canister.
6. Install canister, connect hoses and battery ground cable.