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Purging

Purging


Fuel which evaporates in the tank is passed through a pipe to the EVAP canister. The active charcoal in the EVAP canister becomes saturated as it absorbs the hydrocarbon vapours. When the engine starts, ambient air is drawn through the EVAP canister via the purge valve and a non-return valve in the intake manifold. The fuel vapours follow along with the air and are burned in the engine. When the pressure in the intake manifold exceeds barometric pressure, another non-return valve opens and the vapours are now purged to the suction side of the turbocharger.

When not supplied with current, the purge valve is closed. It is supplied with current from the main relay and controlled with a 8 Hz PWM signal from control module pin 27.

The flow is controlled by the pulse ratio so that it always constitutes a specific proportion of the total flow consumed by the engine.

If the flow has a fuel/air ratio other than 14.7:1, the closed loop control system will be affected. However, since it is not the closed loop system's function to correct for the additional purge air, the purging function has a correction factor which is affected by the closed loop system as soon as purging starts. The closed loop system's entire departure from 1.00 is added to the purge correction factor, which means that the closed loop system fluctuates round 1.00 (0%) even if the purge air contains large quantities of hydrocarbons or consists of clean air.

When purging is not active, the factor 1.00 is used and the entire fuel fault corrected by the closed loop system and the multiplicative and additive adaptations.

The limits for purging adaptation are 0.75 and 1.25.


The following conditions must be fulfilled for purging to be engaged:



• The closed loop system must be active.

• No fuel adaptation in progress; this takes place for 30 seconds every 5 minutes.

• Engine coolant temperature must exceed 50 °C.


The diagnostic tool shows 25% when the correction factor is 1.25 and -25% when the correction factor is 0.75.




Diagnostics


• If a break or short-circuit to ground occurs, diagnostic trouble code P0444 will be generated.

• If there is a short-circuit to B+, diagnostic trouble code P0445 will be generated.

Note

Purging adaptation can give rise to problems when diagnosing faults in the fuel system since the closed loop system can fluctuate round 0% despite major faults such as air leakage. If an LED is connected instead of the purge valve, and a purging adaptation reading obtained with the diagnostic tool, the total fuel fault when the purge is active will be shown provided that the multiplicative and additive adaptations are first zeroed.