Evaporative Emission Control Canister: Testing and Inspection

TWIN CANISTER SYSTEM LEAK TEST
The ECM runs a diagnostic leak test once every engine start/stop cycle, provided the following conditions occur:
- from "cold", the engine has warmed up to within the normal running temperature range
- Ga period of time (currently 15 minutes but subject to change) has elapsed since ignition ON
- the fuel vapor concentration in the evaporative loss system is not excessive
- the car is stationary with the engine at idle RPM after a driving cycle which included a minimum of approximately 5 minutes at a steady speed
- the fuel tank is between 10 and 90 % full. During the test, the ECM checks that the required fuel tank pressure changes occur in response to the opening and closing of the purge valve and the canister close valve. It also monitors the canister close valve and the fuel tank pressure sensor for circuit continuity and correct signals.

Test Sequence
When the required conditions occur, the ECM begins the test by ensuring that the purge valve is closed. A few seconds later the canister close valve is also closed, causing system vapor pressure to increase. This condition is maintained for approximately 5 seconds, after which the ECM begins to progressively open the purge valve. System vapor pressure then begins to decrease, due to the depression in the intake manifold. At approximately 2.5 kPa (0.36 psi) below ambient pressure, the ECM closes the purge valve. Approximately 5 seconds later, the ECM opens the canister close valve, system pressure returns to ambient and the test is complete.
Total test duration is approximately 25 to 30 seconds.

The test is discontinued if any of the following occur:
- no pressure decrease after the purge valve is opened
- excessive fuel vapor concentration in the system
- engine speed changes from idle

Faults
Any faults detected during the test are logged as Diagnostic Trouble Codes (DTC) in the ECM memory.