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Purging

Purging


The fuel that evaporates in the tank passes through a pipe to the EVAP canister. The active charcoal in the EVAP canister will eventually become saturated as it absorbs the hydrocarbon vapour. When the engine is started, the surrounding air is drawn through the EVAP canister via a purge valve and a nonreturn valve in the intake manifold. The petrol vapour is drawn with the air and burned in the engine. When the pressure in the intake manifold exceeds the atmospheric pressure, another nonreturn valve opens and purging will now take place to the suction side of the turbocharger.

In a de-energized state, the purge valve is closed. It is powered from the main relay and is controlled with an 8 Hz PWM from control module pin 27.

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

If the air/fuel ratio in the flow deviates from 14.7:1, the closed loop will be affected. However, it is not the task of closed loop to correct for the excess caused by the purge so the purge has a correction factor that is affected by the closed loop as soon as the purge starts. The entire closed loop deviation from 1.00 is transferred to the purge correction factor, which means that closed loop fluctuates around 1.00 (0%) even if the purge contains large amounts of hydrocarbons or consists of pure air.

When the purge is inactive, a factor of 1.00 is used and the entire fuel fault is corrected by the closed loop and the multiplicative and additive adaptations.

The limits for purge adaptation are 0.75 and 1.25 respectively.


The following conditions must be fulfilled before purging can be activated:



• Closed loop 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 there is an open circuit or short circuit to ground, diagnostic trouble code P0444 will be generated.

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

Note

Purge adaptation can cause problems during fault diagnosis of the fuel system as closed loop can fluctuate around 0 % despite major faults such as air leaks. The total fuel discrepancy can be shown by reading the purge adaptation with the diagnostic tool while purge is active and an LED is connected in place of the purge valve, provided the multiplicative and additive adaptation have first been zeroed.