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 vapours from the tank and then release them 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 vapours with it into the intake manifold.

EVAP Canister Purge Valve
The purge valve consists of a solenoid valve mounted on top of the evaporative emission canister. The valve is in turn connected to 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 60°C (140°F).

When starting with a hot engine, the temperature of which is above 60°C (140°F), there will be a delay of as long as 30 seconds before the purge valve starts.

The control module starts the purging phase with extremely short ground pulses, the length of which is slowly 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.