Evaporative Emissions System: Description and Operation

Evaporative Emission Canister
The evaporative emission canister consists of a container filled with activated charcoal, the purpose of which is to temporarily store fuel vapors from the tank and then release the vapors via the purge valve to the intake manifold for combustion.

The evaporative emission canister accordingly has hose connections to both the fuel tank and the intake manifold. It is also connected to the surrounding air which is drawn in via this connection and passed through the canister, carrying the fuel vapors with it into the intake manifold.

EVAP Canister Purge Valve
The purge valve consists of a solenoid valve mounted on the hose between the emission canister and the intake manifold.

The valve, which receives its power supply from the main relay, is controlled from pin 61 of the control module with 7.5, 15 or 30 Hz, depending on engine load/rpm. The control module activates the purge valve as soon as the closed loop system is activated and engine temperature rises above 40°C (93°F).

When starting with a hot engine, the temperature of which is above 40°C (93°F), there will still be a delay of 30 seconds before the purge valve starts.

The control module starts the purging phase with extremely short ground pulses, the length of which is gradually increased. This is because the closed loop system needs time to compensate for the extra fuel.

After starting, the valve operates for 220 seconds and is then idle for 130 seconds. After this it does not operate at fixed intervals but, depending on the control module's fuel adaptation, may operate continuously for up to 10 minutes.





Fuel Tank Pressure Sensor
A differential pressure sensor is mounted on the fuel tank's filler pipe.

The fuel tank pressure sensor is connected to the filler pipe by means of a short hose and is supplied with 5 V from pin 53 of the control module and grounded via pin 71.

Depending on the difference in pressure between the fuel tank and the atmosphere, the pressure sensor applies a proportional voltage to pin 72 of the control module.

When there is no difference in pressure, the voltage from the pressure sensor is about 2.5 V.

The pressure sensor is used only for OBD II diagnosis.

Shut-Off Valve
The shut-off valve is connected to the evaporative emission canister by means of a hose. The valve is connected to the evaporative emission canister's air supply connection and supplied with power from the main relay. It is connected to ground via pin 65 of the control module.

The valve is open when no voltage is applied to it (zero voltage state).

When the engine is running the valve closes only when a fuel tank integrity check is performed.

The shut-off valve is used only for OBD II diagnosis.,