Emission Control Systems: Description and Operation

Image Legend:
1 = Evaporative Emission Canister Purge Valve
2 = Evaporative Emission (EVAP) Canister
3 = Purge air
4 = Fuel tank




Image Legend:
A = Opening diameter of the Emission Canister Purge Valve over time
t = time




Image Legend:
1a = Fresh air via EVAP canister
1b = Fuel vapor via EVAP canister
A = Diagnostic threshold

Function
Monitoring of the fuel tank ventilation system (EVAP Control System) is based on the recognition, that the oxygen sensor in the ECM adjusts to a leaner operation with additional fuel from the Evaporative Emission (EVAP) Canister through the opened Emission Canister Purge Valve, or with fresh air via the purge line of the (EVAP) Canister adjusts to richer operation. Also the duty cycle of the Idle Air Control (IAC) Valve is changed.




Image Legend:
Tv = Duty cycle Idle Air Control (IAC) Valve
A = Diagnostic threshold
t = Time

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

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

Should the additional fuel/air mixture from the EVAP Purge Valve (fuel vapor and fresh air) be exactly at the ratio of Lambda = 1, the duty cycle of the AC Valve is reduced. When the diagnostic threshold is reached also here, the diagnosis is successfully concluded.

Prerequisites for the start of the diagnosis
- Engine at idle (Throttle closed)
- Vehicle speed = 0 km/h (speed signal)
- Time after starting engine > 1000 S (clock in ECM)
- Oxygen sensor activated
- No other faults

NOTE: The tank ventilation diagnosis can be initiated via a special function in the scan tool. With that the waiting time is omitted after engine start up. All other conditions however must be met.