Fuel Tank Pressure Control Valve
WARNING: This page is about a different car, the 1997 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
Apply approximately 15 in. Hg to fuel tank pressure control valve. The diaphragm should hold vacuum for at least 20 seconds. If fuel tank pressure control valve does not hold vacuum, replace tank pressure control valve.