Evaporative Purge System Monitoring

Evaporative system monitoring structure





OBD-II Evaporative System









Tables of OBD Requirements for Evaporative Purge System Monitoring

GENERAL DESCRIPTION

The evaporative system monitoring permits the detection of leaks in the evaporative emission control system with a diameter of 1 mm and up.

The evaporative emission control system is hermetically sealed off from the atmosphere by means of a shut-off valve at the charcoal canister. When the purge valve opens, the engine generates a vacuum in the tank. After closing the purge valve the pressure increase is measured by a tank pressure sensor. If the vacuum increases above a defined threshold a leak is detected.

As part of the vacuum check the purge flow is verified.

MONITORING STRUCTURE

To start the diagnosis function several conditions have to be satisfied.

- no malfunction related to vehicle speed, coolant and intake air temperature, idle speed, oxygen sensor, purge valve, tank pressure sensor, shut-off valve, throttle position
- coolant temperature > 80°C
- altitude < 2,500 m
- canister purge time > 360 sec after start
- canister purge time between 10 and 150 sec depending on canister saturation
- vehicle speed = 0
- idle speed
- tank pressure within - 4 hPa and + 4 hPa
- pressure increase due to vapor generation < threshold

threshold depends on tank pressure A at test start.

A(hPa) threshold (hPa)
0 2.5
1.0 2.0
2.0 1.9
3.0 1.5
3.5 1.25
4.0 1.0





Test for vapor generation





Evaporative system diagnosis





OBD-II Evaporative System leak check - fuel tank pressure diagram

For further information please refer to the flow charts.