Function

Evaporative emission (EVAP) system

The EVAP canister absorbs the hydrocarbons in the fuel vapors so that they are not released. The engine control module (ECM) controls the EVAP valve. The hydrocarbons are emptied from the EVAP canister when the valve is open. When emptying, the atmospheric air flows through the EVAP canister (where stored hydrocarbons are gathered) and on to the intake manifold and combustion chamber. The energy in the hydrocarbons is then used during the combustion process. So that emissions are not negatively affected, the EVAP valve is pulsed by a pulse width modulation (PWM) signal, the frequency of which is calculated by the engine control module (ECM)

The signals from the heated oxygen sensors (HO2S) are used as a reference. The EVAP valve is closed during the leak diagnostic. The leak diagnostic unit checks the gas and fuel filled sections of the EVAP system. Any leaks in the fluid filled sections cannot be detected. When the pump is not activated, it is open so that the fuel tank and EVAP system can "breathe". A number of parameters must be met before the EVAP canister can be emptied. For example:
- the engine coolant has reached a certain temperature.
- the fuel/air ratio is within the tolerance zones.

During leak diagnostics, the control module activates the pump and test pessurizes/calibrates the pump against a built-in leak scenario in the pump module. A valve then closes so that the pump motor can create overpressure in the tank and EVAP system. The control module is able to determine the size of the leak in the tank and EVAP system by measuring the power consumption of the pump in these two circumstances. If the current level is too low in relation to the level for the in-built leak scenario, there is leakage in the system. The built-in PTC heater element is used to prevent condensation in damp weather which could disrupt the leak diagnostic.