Evaporative Check Valve: Description and Operation

Evaporative Emission Control System:

An evaporative emission control system is used to prevent emission of fuel vapor.

The vapor generated in the fuel tank while driving or the engine at a stop passes through a 2-way check valve and enters the charcoal canister where the charcoal absorbs and stores the fuel vapor.

The canister purge VSV is controlled by ECM according to signals from various sensors.

Only when the following conditions are all satisfied, ECM opens vacuum passage of VSV.

- When engine cooling water temp. is higher than 60° C (140° F)
- When engine speed is higher than 1500 rpm.
- When throttle valve opens wider than idle position (Idle switch OFF)
- When engine is running with the load higher than specified.

As a result, fuel vapor in the canister is sucked into intake manifold.

In this state, the canister is purged or cleaned by air drawn through the filter at the bottom of the canister.

The 2-way check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow into the charcoal canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let the air flow into the fuel tank.