General

Evaporative Emission Control Operation
Fuel vapor is stored in the activated charcoal (EVAP) canister for retention when the vehicle is not operating. When the vehicle is operating, fuel vapor is drawn from the canister into the engine via a purge control valve. The vapor is then delivered to the intake plenum chamber to be supplied to the engine cylinders where it is burned in the combustion process.

During fuel filling the fuel vapor displaced from the fuel tank is allowed to escape to atmosphere, valves within the fuel filler prevent any vapor escaping through to the EVAP canister as this can adversely affect the fuel cut-off height. Only fuel vapor generated whilst driving is prevented from escaping to atmosphere by absorption into the charcoal canister. The fuel filler shuts off to leave the tank approximately 10% empty to ensure the ROVs are always above the fuel level and so vapor can escape to the EVAP canister and the tank can breathe. The back pressures normally generated during fuel filling are too low to open the pressure relief valve, but vapor pressures accumulated during driving are higher and can open the pressure relief valve. Should the vehicle be overturned, the ROVs shut off to prevent any fuel spillage.

Fuel vapor generated from within the fuel tank as the fuel heats up is stored in the tank until the pressure exceeds the operating pressure of the two-way valve. When the two-way valve opens, the fuel vapor passes along the vent line from the fuel tank (via the fuel tank vapor separator) to the evaporation inlet port of the EVAP canister. The fuel tank vents between 5.17 and 6.9 kPa.

Fuel vapor evaporating from the fuel tank is routed to the EVAP canister through the fuel vapor separator and vent line. Liquid fuel must not be allowed to contaminate the charcoal in the EVAP canister. To prevent this, the fuel vapor separator fitted to the fuel neck allows fuel to drain back into the tank. As the fuel vapor cools, it condenses and is allowed to flow back into the fuel tank from the vent line by way of the two-way valve.

The EVAP canister contains charcoal which absorbs and stores fuel vapor from the fuel tank while the engine is not running. When the canister is not being purged, the fuel vapor remains in the canister and clean air exits the canister via the air inlet port.

The engine management ECM controls the electrical output signal to the purge valve. The system will not work properly if there is leakage or clogging within the system or if the purge valve cannot be controlled.

When the engine is running, the ECM decides when conditions are correct for vapor to be purged from the EVAP canister and opens the canister purge valve. This connects a manifold vacuum line to the canister and fuel vapor containing the hydrocarbons is drawn from the canister's charcoal element to be burned in the engine. Clean air is drawn into the canister through the atmosphere vent port to fill the displaced volume of vapor.

The purge valve remains closed below preset coolant and engine speed values to protect the engine tune and catalytic converter performance. If the EVAP canister was purged during cold running or at idling speed, the additional enrichment in the fuel mixture would delay the catalytic converter light off time and cause erratic idle. When the purge valve is opened, fuel vapor from the EVAP canister is drawn into the plenum chamber downside of the throttle housing, to be delivered to the combustion chambers for burning.

The purge valve is opened and closed in accordance with a pulse width modulated (PWM) signal supplied from the engine management ECM. The system will not work properly if the purge valve cannot be controlled. Possible failure modes associated with the purge valve are listed below:
^ Valve drive open circuit.
^ Short circuit to vehicle supply or ground.
^ Purge valve or pipework blocked or restricted.
^ Purge valve stuck open.
^ Pipework joints leaking or disconnected.

Possible symptoms associated with a purge valve or associated pipework failure is listed below:
^ Engine may stall on return to idle if purge valve is stuck open.
^ Poor idling quality if the purge valve is stuck open
^ Fuelling adaptions forced excessively lean if the EVAP canister is clear and the purge valve is stuck open.
^ Fuelling adaptions forced excessively rich if the EVAP canister is saturated and the purge valve is stuck open.
^ Saturation of the EVAP canister if the purge valve is stuck closed.

To maintain driveability and effective emission control, EVAP canister purging must be closely controlled by the engine management ECM, as a 1 % concentration of fuel vapor from the EVAP canister in the air intake may shift the airfuel ratio by as much as 20%. The ECM must purge the fuel vapor from the EVAP canister at regular intervals as its storage capacity is limited and an excessive build up of evaporated fuel pressure in the system could increase the likelihood of vapor leaks. Canister purging is cycled with the fuelling adaptation as both cannot be active at the same time. The ECM alters the PWM signal to the purge valve to control the rate of purging of the canister to maintain the correct stoichiometric airfuel mixture for the engine.