Evaporative Check Valve: Description and Operation

When the engine is not running:
The fuel vapor taken up into the rollover fuel trap is fed into the check and relief valve. When the pressure of the fuel vapor reaches 40 mmHg (1.6 in.Hg.), the check valve opens, allowing the vapor into the EVAP canister.

When the engine is running:
When negative pressure is developed in the fuel tank and the difference of pressure between the relief side and fuel tank becomes 20 mmHg. (0.8 in.Hg.), the relief valve opens, allowing ambient air from the air cleaner into the fuel tank. This ambient air replaces the vapor within the fuel tank, bringing fuel tank back into an atmospheric pressure condition.