Leakage Diagnostic
Leakage Diagnostic
Vapor that evaporates from the fuel in the fuel tank is routed to and stored in the EVAP canister from where it is introduced into the combustion process via the Canister Purge (CP) valve.
A leak diagnostic has been introduced in certain markets to ensure that there are no leaks in the fuel tank system. The diagnostic is designed to detect leakage corresponding to a 0.20 inch or larger hole. The fuel tank system consists of fuel tank, fuel filler pipe, EVAP canister, CP valve and all pipes between these components. To be able to diagnose the fuel tank system, it is also equipped with a diagnostic module (DMTL = Diagnostic Module Tank Leakage) including the electrical driven air pump.
The diagnostic is divided into different phases as follow:
Reference leak measurement, performed every LD Rough leak test, performed every DCY
Small leak test performed every second DCY when enabling conditions are met.
The diagnostic is performed by measuring the LD Pump Module Motor current and then compares it to a specified reference current. If a fault is detected in any of the phases the diagnostic is interrupted and the DTC for the component identified is stored. Diagnosis is carried out in the following stages: At the first engine stop after refueling, the module DMTL will start if conditions are met (conditions for soak time and fuel level are over ridden). When the fuel level sensors are working correctly and the fuel level is higher than 85% or smaller than 15% all leakage tests are aborted. Also, the test is aborted if the initial rate of change is higher than a calibrated level due to a combination of high fuel level and high evaporation. In case of healing attempt the test is aborted when the fuel level is too high, which is calibrated lower than 85%. While the fuel level sensors are not working correctly the test only will be aborted if the initial rate of change is higher than a calibrated level.
1. Reference leak measurement phase
For the reference current measurement, the motor pump is switched on. In this mode fresh air is pumped through a 0.02-inch reference orifice, situated internally in the module, and the pump motor current is measured. At some unusual operating conditions the pump current may not stabilize. In this case the leak check is aborted and a new leak check will be performed in the next after run. To prevent a permanent disablement of the leak check due to a DM-TL module problem, the number of subsequent irregular current measurements is counted and a module error is set as soon as the counter exceeds a calibrated value.
2. Rough leak test phase
In this monitoring mode the changeover valve is switched over (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-inch) is performed by monitoring the pump motor current gradient. Relative pump motor current is created by using minimum pump motor current and reference pump motor current. Area ratio is created by dividing integrated relative current with ideal area, which is the linear integrated area from minimum pump current to current sample of the current. If the relative current has increased above an upper limit but not exceeded a calibrated area, within a calibrated time, the rough leak check has passed without a fault. If the calibrated area ratio is reached before the relative pump current limit, within the calibrated time, a rough leak fault code is set. The integrated relative pump current area A-int is defined by;
A-int = A1 + A2
and the ideal area A-ideal,
A-ideal = A2.
See figure below.
3. Small leak test phase
If the conditions for a small leak check ((3) 0.02- inch) are set the pump motor remains on in monitoring mode until an elliptic combination of the relation pump current and area ratio are fulfilled, or a maximum time limit has been reached. The judgment is based on a test value which is a combination of the actual area ratio and gradient of area ratio with respect to relative pump current. If the estimated leak size is close to the fault limit (0.020 inch leaks) the monitor may decide to extend the run time of the pump to increase the build up pressure. This will make the judgement of a small leak safer.
If the test value is very near to set 0.02 inch leakage the reference leak measurement phase is performed again in order to compensate test value and make a final judgment. If the motor current decreases or increases too much during one of the tests, the test is aborted and a new leak test will be performed in the next after run.
Monitoring conditions
To carry out the leak diagnostic it is necessary that:
- Engine-on time is at least 20 minutes and last engine off time is more than 5 hours.
- ECM (=Engine Control Module) is in after run mode
- Engine speed is 0 rpm
- Vehicle speed is 0 km/h
- Altitude is less than (or equal to) 2500 meters
- Engine coolant temperature is above (or equal to) +4 Degrees C
- Ambient temperature is between +4 Degrees C and +35 Degrees C
- Fuel level between 15% to 85% when no fuel level fault
- Fuel level is not used if fault on fuel level
- Rate of change of the initial relative pump current is low enough
- Concentration of fuel vapor in the EVAP canister is not excessive
- Battery voltage between 11.0 V and 14.5 V
- Purge valve is closed.
With the following errors the leakage detection monitoring can not be performed. These errors will therefore disable the leakage detection monitoring and the MIL (and the corresponding fault code) will be set. The disable conditions are:
- Error on power stages DM-TL pump
- Error on power stage purge valve
- Error on purge valve
- Error on change-over valve
