Leak Detection Normal Operation
In its inactive state, filtered fresh air enters the evaporative system through the sprung open valve of the tank leakage detection module. When the ECM activates the tank leakage detection module for leak testing, it first activates only the pump motor. This pumps air through a reference orifice 0.5 mm (0.02") which causes the electric motor to draw a specific amperage value. This value is then equivalent to the size of the reference orifice. The solenoid valve is then energized which seals the EVAP system and directs the pump output to pressurize the EVAP system. See Fig 1.
The system is detected as having a large leak if the amperage value is not realized, a small leak if the same reference amperage is realized or no leak if the amperage value is higher than the reference leak value.
The leak detection module ensures accurate fuel system leak detection for leaks as small as 0.5 mm (0.02"). The pump contains an integral DC motor which is activated directly by the engine control module. The ECM monitors the pump motor operating current as the measurement for detecting leaks.
In its inactive state the pump motor and the change over valve of the DC Motor LDP are not energized. When purge valve operation occurs filtered air enters the fuel system compensating for engine vacuum drawing on the hydrocarbon vapors stored in the charcoal canister.
The ECM only initiates a leak diagnosis test every second time the criteria are met. The criteria are as follows:
- Engine Off With Ignition Switched Off
- Engine Control Module still in active state or what is known as "follow up mode" (Main Relay energized, control module and ECM components on-line for extended period after key off).
- Prior to Engine/Ignition switch OFF condition, vehicle must have been driven for a minimum of 20 minutes.
- Prior to minimum 20 minute drive, the vehicle must have been OFF for a minimum of 5 hours.
- Fuel Tank Capacity must be between 15 and 85% (safe approximation between 1/4-3/4 of a tank).
- Ambient Air Temperature between -7°C & 35°C (20°F & 95°F).
- Altitude < 2500 m (8,202 feet)
- Battery Voltage Between 11.5-14.5 Volts
When these criteria are satisfied every second time, the ECM will start the Fuel System Leak Diagnosis Test. The test will typically be carried out once a day i.e. once after driving to work in the morning, when driving home in the evening the criteria are once again met but the test is not initiated. The following morning, the test will run again.