Engine Not Running
Fuel tank pressure is maintained by the vent valve (a pressure/vacuum relief valve). When engine is off and vapors expand, the pressure/vacuum relief valve in vent valve opens, allowing fuel vapors to escape to the charcoal canister.
As fuel cools down, volume is reduced, creating a vacuum in fuel tank. Below a present value, the vacuum portion of vent valve opens, allowing air and/or fuel vapors from the canister to travel to the fuel tank and reduce the 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.
The breathing valve will compensate for any condition of less-than-atmospheric pressure in the fuel tank (due to decrease of fuel temperature or normal use of fuel from the tank). Valve opens and allows outside air to enter system.
The float chamber vent valve (carburetor models only) is a solenoid operated float chamber vent valve that is incorporated in the carburetor. This valve is operated by turning the ignition on or off. When the ignition switch is off, no current is received by the solenoid valve, allowing a spring to press the valve disc against the upper valve seat. This allows fuel vapors from the float chamber to be vented to the charcoal canister. When the ignition switch is turned on, the solenoid pushes the valve stem and disc against the lower valve seat. This closes the external vent to charcoal filter and the carburetor is vented internally. See Figure.