Fuel Evaporation System: Operation
When the engine is not running, fuel tank vapors flow through vent line into canister for storage. A check valve in canister's purge port prevents vapors from flowing out of canister through purge line.
When engine speed increases to a moderate level, throttle valve opens purge port to manifold vacuum and canister vapors flow through purge line into combustion chambers for burning. When the throttle valve closes, it shuts off manifold vacuum to purge port and canister purging stops.
Van uses an additional check valve located in vent line between fuel tank and canister to prevent fuel leakage if van is rolled over.