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bowhunter72: The purge valve is on the firewall, next to the brake booster, beneath the grey and black electrical connectors (which are for the upstream and downstream oxygen sensors on the firewall side of the engine). It's connected to the black vacuum hose entering the top of the intake manifold behind the throttle body.
When the car is parked during the day, as outdoor temperatures rise, fumes expand inside the gas tank. To prevent air pollution, the gas fumes are absorbed by a charcoal canister.
When the engine runs, the purge valve opens and pulls a vacuum on the charcoal canister, sucking trapped fumes into the engine, making room in the canister for tomorrow's fumes.
This is why emissions tests include checking the gas tank cap for a tight seal. And it's why we're not supposed to top off the gas tank when fueling up - you don't want liquid fuel to overload the charcoal canister & vapor purge system.
