Canister Purge Control Valve: Description and Operation

Evaporative Valve

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 engine operating conditions 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 fueling 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.

The purge valve is mounted in the engine compartment on the left-hand bulkhead and is controlled by the engine control module (ECM). It is linked to the single carbon canister system located on the left-hand-side of the vehicle, rearward of the front road wheel.

The system comprises a single carbon canister/bracket and Rochester valve.





Operation
Under control of the ECM the purge valve flow rate is monitored/modified to maintain the correct AFR and avoid canister saturation.

Monitoring Procedure
This diagnostic operates during the first long duration hot idle period after engine start and works by gradually opening the purge valve and monitoring changes in the following;air fuel ratio (AFR) closed loop control value, idle speed control feedback and engine RPM. If none of these variables changes by more than a predetermined value the purge flow failure is made.

The purge valve drive circuit is monitored during driving operation; if the driver state does not match the command state for over a predetermined number of on/off operations then a failure judgement is made.