Fuel Tank Pressure Release Valve: Description and Operation

Vacuum/pressure Relief Valve
The flow of vapor to the canister is controlled by the vacuum/pressure relief valve (Rochester valve). The valve has three functions: fuel tank pressure relief, fuel tank vacuum relief, and vapor flow control.

When the vehicle is not operating and fuel temperature in the tank is increasing, vapor is released, limiting the tank pressure to approximately 1 to 1.25 psi. When the vehicle is not operating and fuel temperature in the tank is decreasing, a negative pressure is created. The umbrella check valve in the vacuum/pressure relief valve allows vapor to flow back into the tank from the charcoal canister, preventing the tank from collapsing. When the engine is operating, a manifold vacuum signal, from the intake manifold, opens the valve allowing vapor flow to the canister and reducing the fuel tank pressure to near zero. An additional fuel tank overpressure safeguard is provided by a 4 psi relief valve in the fuel cap.