See Figure 1
The thermal vent valve 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. This effect may occur 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 can not enter the carburetor fuel bowl vent (now open), but must be routed to the carbon canister. As the engine compartment warms up during normal engine operation, the thermal bowl vent opens. When the engine is turned off again, the fuel vent valve or solenoid vent valve opens, allowing vapors to flow through the open thermal vent valve and into the carbon canister. As the thermal vent valve cools, it again closes and the cycle begins again.
Inspect the valve for air flow and compare to the desired results at the specific temperature. At 90°F, the vent valve should be fully closed, and at 120°F, the vent valve is fully open. If the performance of the valve differs from the desired results, replace the valve.