Tank pressure control valve is a vacuum regulated/pressure control valve located in fuel tank or in vapor delivery hose between fuel tank and carbon canister. When engine is not running and tank pressure is less than .9 psi (.63 kg/cm 2 ), internal spring pressure holds valve in the closed position.
This causes fuel tank low-pressure vapors to be vented through a restriction in valve. This restriction will retain most fuel tank vapors in fuel tank. When tank pressure rises and overrides spring tension, fumes are vented to the carbon canister. When engine is running, vacuum is applied to upper port of valve, opening passage between fuel tank and carbon canister, which is purged by engine vacuum.