Carburetor Fuel Bowl Thermal Vent Valve

The Thermal Vent Valve, Figure 1 is a temperature actuated Off/On valve. It is inserted in the carburetor-to-canister vent line and is closed when the engine compartment is cold. This prevents fuel tank vapors (generated when the fuel tank heats up before the engine compartment does) from being vented through the carburetor fuel bowl-forcing them instead into the carbon canister.

This effect can occur, for instance, when sunlight strikes a vehicle which has been sitting out all night, and begins to warm the fuel tank. With the thermal vent valve closed, the vapors cannot enter the carburetor fuel bowl vent valve (now closed) but must be routed to the carbon canister. As the engine compartment warms up, during normal engine operation, the thermal vent valve opens. When the engine is again turned off, the thermal vent valve (now open because underhood temperature is above 120°F) allows fuel vapors generated from the carburetor float bowl to pass through the valve and store themselves in the carbon canister. As the thermal vent valve cools, it again closed and the cycle begins again.