Fuel Fill Pipe
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The fuel fill pipe has a built-in restrictor in order to prevent refueling with leaded fuel. Once the fill vent is obstructed, fuel backs up the fill pipe and trips the dispensing nozzle. The fuel tank vent valves are connected and route to the canister to collect hydrocarbon emissions during operation of the vehicle.