Evaporative Emissions System: Description and Operation

Evaporative Emission (EVAP) Control Diagram




The EVAP controls minimize the amount of fuel vapor escaping to the atmosphere. Vapor from the fuel tank is temporarily stored in the EVAP canister until it can be purged from the canister into the engine and burned.
- The EVAP canister is purged by drawing fresh air through it and into a port on the intake manifold.

The purging vacuum is controlled by the EVAP canister purge valve, which operates whenever engine coolant temperature is above 149 °F (65 °C).

- When vapor pressure in the fuel tank is higher than the set value of the EVAP two way valve, the valve opens and regulates the flow of fuel vapor to the EVAP canister.
- During refueling, the fuel tank vapor control valve opens with the pressure in the fuel tank, and feeds the fuel vapor to the EVAP canister.

EVAP Canister

The EVAP canister temporarily stores fuel vapor from the fuel tank until it can be purged back into the engine and burned (refer to the System Diagram to see the functional layout of the system).

EVAP Canister Purge Valve

When the engine coolant temperature is below 149 °F (65 °C), the ECM/PCM turns off the EVAP canister purge valve which cuts vacuum to the EVAP canister.

Fuel Tank Pressure (FTP) Sensor




The FTP sensor converts fuel tank absolute pressure into an electrical input to the ECM/PCM.

EVAP Two Way Valve and EVAP Bypass Solenoid Valve




The EVAP two valve is installed between the fuel tank and the EVAP canister line.

The EVAP two way valve sends fuel vapor to the EVAP canister corresponding to the pressure inside the fuel tank and vacuum, and it is returned in the fuel tank. The EVAP bypass solenoid valve opens to bypass the two way valve when doing the EVAP leak check.