Evaporative Emissions System: Operation
When the engine is not running, fuel vapors that expand in the fuel tank pass through the expansion tank, check valves, and into the charcoal canister. Carburetor vapors are also routed to the canister when the engine is stopped or idling. When the throttle is moved off idle, the vent valve solenoid is energized and float bowl is vented to atmosphere.
At low engine loads, a ported vacuum line opens the canister purge valve, and intake manifold vacuum pulls the stored vapors into the air cleaner and intake manifold. When engine load is high or throttle wide open, vapors are routed through the intake manifold primarily.
The check valves in the expansion tank and fuel lines allow replacement air to enter the fuel tank as fuel is removed, and prevent fuel from escaping the tank should the car roll.