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Fuel evaporation

Fuel evaporation



Evaporative emission canister

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

The evaporative emission canister thus has a hose connection both to the fuel tank and to the intake manifold, but is also connected to the surrounding air. Air is drawn via this connection with the surrounding air through the evaporative emission canister and brings fuel vapours into the intake manifold with it.

EVAP canister purge valve

The purge valve consists of a solenoid valve mounted on the vacuum hose between the evaporative emission canister and intake manifold after the throttle.

The valve is supplied with current from the main relay and controlled from pin 27 of the electronic control module. On cars without tank integrity diagnosis, the lead is connected to pin 21 of the electronic control module. By measuring the voltage on pin 21, the electronic control module determines whether the function is electrically correct.

The valve is supplied with PWM at a frequency of 8 Hz and starts to operate as soon as closed loop is activated, engine coolant temperature is above 60°C (140°F) and the pressure in the intake manifold is below 90 kPa.

The electronic control module starts purging with extremely short ground pulses, the duration of which slowly increases. The reason for this is that the closed loop must be given time to compensate for the additional fuel. The purge valve operates for 4 1/2 minutes, stops for 1/2 a minute, operates for 4 1/2 minutes, and so on.

Tank integrity diagnosis (M96 1/2 and later)



Pressure sensor

A differential pressure sensor is mounted on the fuel tank filler pipe. The pressure sensor is connected to the filler pipe by means of a short hose and supplied with 5 V from pin 42 of the electronic control module. It is grounded via pin 67 of the electronic control module.

Depending on the difference in pressure between the tank and the atmosphere, the pressure sensor applies a proportional voltage to pin 21 of the electronic 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 solely for OBD II diagnosis.

Shut-off valve

The shut-off valve is mounted adjacent to the evaporative emission canister. The valve is connected to the canister's air supply connection and supplied with current from the main relay. It is grounded via pin 28 of the electronic control module.

The valve is open when no current is applied to it.

The valve is open when no current is applied to it. The shut-off valve is used solely for OBD II diagnosis.