Evaporation System Engine Not Running
Fuel tank pressure is maintained by the vent valve, which is a pressure/relief valve. When engine is off and vapors expand (heated by sun), the pressure relief valve in vent valve opens, allowing fuel vapors to escape to charcoal canister.
As fuel cools down, volume is reduced, creating a vacuum in fuel tank. Below a preset value, the vacuum portion of the vent valve opens, allowing air and/or fuel vapors 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.