How the EVAP System Works

Here's how the evaporative emissions and fuel system work on your car.

The EVAP system is responsible for venting harmful gasoline vapours from the gas tank into the atmosphere, causing smog.

It consists of the gas tank, filler neck, cap, vacuum lines, switching valves and the charcoal canister.

When the tank is being refuelled, the fill check valve is opened to vent building fumes in the tank to the charcoal canister. The charcoal absorbs the vapour, and will release it into the engine through the purge valve so it can get burned.

When the tank is full, the fill check valve has a float that rises and cuts off the vent line through a diaphragm. This prevents liquid fuel from entering the charcoal canister.

The vacuum switching valve allows the ECU to pull and hold a vacuum on the evap system for self diagnostic. Using the vapour pressure sensor, it can determine if there are leaks and throw a check engine light.

These self diagnostics are part of every car with OBD2, and are essential for an emission test to pass.

The fuel tank is also cut open, exposing the baffles inside, as well as the high and low points of the tank and inlet check valve.

Vacuum lines in between the gas tank, charcoal canister and air filter ensure air is directed for proper venting, purging and self diagnostic.

Care must be taken when working on the fuel system, as gasoline vapours are extremely flammable.