Leakage Detection Procedure

In switched off condition the fuel tank breathes through the canister, the changeover valve and the air filter. For purging the canister, the purge control valve is opened and fresh air-flows via air filter, changeover valve and canister into the intake manifold

For a reference current measurement the motor pump is switched on for a small period of time while the changeover valve and purge control valves are closed. In this mode fresh air is pumped through the 0.02 inch reference orifice and the pump motor current is measured. At some unusual operating conditions the pump current may not stabilize. In this case an optional 0.02 or 0.04 inch leak checks demand will not be performed (delay to the next monitoring event). To prevent a permanent disablement of the leak check due to a DM-TL module problem, the number of subsequent irregular current measurements are counted and a module error is set as soon as the counter exceeds a calibrated value.

In the following monitoring mode the change over valve is switched on (the purge control valve remains closed). The motor current drops to a zero load level. Fresh air is now pumped through the canister into the tank. This creates a small overpressure at a tight evaporative system which leads to a current increase.

The rough leak check (> 0.04 in) is performed by monitoring the pump motor current gradient. Therefore the current is measured after a calibrated time. If the difference in the current is above an upper limit the rough leak check has passed without a fault. If this high gradient is not detected (at lower fuel tank levels), a second gradient measurement is performed by a longer measurement.

If the conditions for a small leak check (> 0.02 in) are set, the pump motor remains on (monitoring mode) until its current exceeds the reference current or until the motor current gradient is closed to zero (current stabilizes below the reference current). If the motor current exceeds the reference current, the system is monitored as tight. In case of stabilizing below, the system is assumed as untaught. In this case the stabilized current is compared against a calibrated limit to prevent the detection of a small leak fault in case of an opened fuel filler cap during the small leak check. If the motor current decreases during one of the checks, the check is aborted and above-mentioned counter is increased.

BASE MONITORING CONDITIONS: 

• ECM in power down mode
• engine speed = 0 RPM
• altitude < 2500 m1)
• engine coolant temperature at start > 4°C
• no error on engine coolant temperature sensor
• ambient temperature in 4°C-35°C
• canister load < close to full canister
• fuel level steady and in range of 15%-85%2)
• engine-on time > 20 min.
• vehicle speed = 0.0 km/h
• no error on vehicle speed sensor
• battery voltage in range of 11.5V-14.5V
• last engine-off time > 5hr
• no error on power stages heated DM-TL pump and valve
• no error on purge valve, including power stage
• above criteria true for the second time

1. altitude adapted from engine load detection (throttle position, air mass flow and engine speed)
2. condition is only used as long as the fuel level sensor is monitored without a fault

CONDITIONS FOR SMALL LEAK CHECK (0.02 IN): 

or . refueling detected (difference in volume > 5L)

or . number of rough leak tests > 14 times since last small leak test

or . 0.02 inch leak detected in previous driving cycle

or . last 0.02 inch leak check disabled by instable motor pump current and last engine off time > 5hr