Engine Not Running
Fuel tank pressure is maintained by the vent valve, which is a pressure/relief valve. When engine is off and vapor expands, the pressure relief valve in vent valve opens, allowing fuel vapor to escape to charcoal canister.
When vacuum in the tank reaches a preset value, the vacuum portion of the vent valve opens, allowing fuel vapor from canister to travel to fuel tank, reducing vacuum. If system malfunction causes fuel tank pressure to increase above 1.5-4.5 psi (.1-.3 kg/cm2 ), relief valve portion of filler cap opens to vent excess pressure.