Canister Purge Control Valve: Description and Operation

Excess vapor formed in the fuel tank is absorbed into the evaporative emission purge control canister. While the engine is running, the fuel absorbed in the canister is gradually purged back into the engine. The rate of purging is governed by the operating conditions of the engine and vapor concentration level. Operating conditions which affect the purge rate are:
- Speed and load.
- Coolant temperature.
- Time elapsed from start up.
- Closed loop fueling.

Determination of the vapor concentration is made by stepped opening of the EVAP valve and subsequent monitoring of the fuelling correction. This function is performed prior to purging, so that at the onset of purging, the EVAP valve can be set to the optimum position. Should the ECM be unable to determine the concentration before purging, a default value is employed, which is then modified whilst purging is in progress.

When the purging process is operational the ECM modifies the basic fueling calculation to maintain the correct air/fuel ratio.

Purging is inhibited during fuel cut-off and stability/traction control intervention.