Active diagnosis

Monitoring of the fuel tank ventilation system is based on the principle that the oxygen sensor in the DME control module adapts the mixture towards lean via the open EVAP canister purge valve when additional fuel flows from the EVAP canister, and enriches the mixture in the case of fresh air coming through the purge air line of the EVAP canister. The duty cycle of the Idle Air Control (IAC) Valve is also changed.

Fig 1: Identifying Operating Diameter Of EVAP Canister Purge Valve

Courtesy of PORSCHE OF NORTH AMERICA, INC.

Fig 2: Identifying Oxygen Sensor Diagnostic Threshold

Courtesy of PORSCHE OF NORTH AMERICA, INC.

The oxygen sensor operates during fresh air supply up to the diagnostic threshold in the direction of enrichment or in the case of fuel vapors up to the diagnostic threshold in the direction of lean operation.

By reaching the diagnostic threshold within a predetermined time the opening of the EVAP canister purge valve is detected and the diagnosis is therefore successfully concluded.

Fig 3: Identifying EVAP Canister Purge Valve Diagnostic Threshold

Courtesy of PORSCHE OF NORTH AMERICA, INC.

Should the additional fuel/air mixture from the EVAP canister purge valve (fuel vapors and fresh air) be exactly at the ratio of lamda = 1, the duty cycle of the IAC valve is reduced. When the diagnostic threshold is reached, here, too, diagnosis is successfully completed.