Its a simple vent hose that passes air into the tank when the engine's running & taking fuel out. Attaching it to the air cleaner allows filtered air to go to the tank especially if you use Fram or AC's tiny filter hookup (which also acts as a flame arrestor in the event of a backfire). It also allows gas fumes to move into the air cleaner on hot days when the engine's not running, so I suppose its possible to smell fumes under some situations. If this bothers anyone, those charcoal cannisters are still available from Ford; they just don't last forever....
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