Leak Diagnostics
All gases that evaporate from fuel in the fuel tank must be led to and stored in the evaporative emission system (EVAP) canister so that they can be directed into the engine for combustion. In order to detect leakages which cause evaporation of gases into the air, the fuel tank system is diagnosed for leakage. The fuel tank system consists of:
- fuel tank
- Canister purge (CP) valve (1)
- EVAP canister (2)
- leak diagnostic unit (3)
- air cleaner (ACL) (4)
- Roll-over valve (5)
- Float Limit Vent Valve (6)
- fuel filler pipe (7)
- all lines between the above components.
The fuel tank system has a leak diagnostic unit to diagnose any leakage. The leak diagnostic unit pressurizes the fuel tank system when the ignition is off, if the conditions for diagnosis have been met. The control module can detect faults in the function of the leak diagnostic unit and leakage that is 0.5 mm or greater. Minor leak; leakage greater than 0.5 mm but less than 1.0 mm. Major leak; leakage greater than 1.0 mm.
The leak diagnostic unit consists of a pump and a valve that controls the air flow in the unit. The fuel tank system tests for leaks by measuring the power consumption of the pump. The power consumption of the pump corresponds to a certain pressure in the fuel tank system. During diagnosis, the rate at which the pressure can build up is checked, taking into account the quantity of fuel in the tank. The quicker the pressurization the better the fuel tank system is sealed.
Conditions for diagnosis
The diagnosis begins when all the following conditions are met:
- There must be no diagnostic trouble code (DTC) stored for the following components or functions:
- the power stage for the pump in the leak diagnostic unit
- the power stage for the valve in the leak diagnostic unit
- the power stage for the EVAP canister purge valve
- the EVAP canister purge valve.
- the engine coolant temperature (ECT) sensor
- atmospheric pressure sensor
- outside temperature sensor.
- The engine is switched off until the engine coolant temperature (ECT) has fallen to a few degrees above the outside temperature, then engine running for at least 10 minutes.
- Ignition off
- Vehicle speed 0 km/h
- Engine coolant temperature (ECT) -5 °C or higher.
- Maximum altitude of 2500 meters above sea level
- Outside temperature between -5 and +35 °C.
- Fuel volume in the tank less than 85%. The engine control module (ECM) ignores these parameters if a diagnostic trouble code (DTC) is stored for the fuel level sensor and the fuel volume cannot be determined.
- Battery voltage between 11.0-15.0 V. The voltage must be stable.
- EVAP canister purge valve closed
- Low volume in the canister.
- Fuel tank filler cap locked. Tip. Locking occurs when the vehicle speed exceeds approximately 20 km/h.
Diagnostic phases
The diagnostic is divided into the following phases and is carried out in sequence when all conditions for the diagnostic have been met.
- reference phase
- function test
- leak diagnostic
Reference phase (1-2)
The illustration is a diagram of a fault free fuel tank system.
Before the leak diagnostic begins, the control module runs reference phase for leakage. Reference phase (1-2) for leakage that is 0.5 mm is carried out by the pump in the leak diagnostic unit pumping ambient air through a 0.5 mm hole and back out to the ambient air. At the same time, the power consumption (A) of the pump is measured and stored in the control module. The stored value (A) for the power consumption of the pump corresponds to a leakage of 0.5 mm. This value is then compared by the engine control module (ECM) to determine the leak status of the fuel tank system.
Function test (1-3)
If the value for the power consumption of the pump is too high or low during reference phase (1-2), or if the value for pump power consumption varies too much during reference phase (1-2), the diagnostic is cancelled and starts again the next time the conditions for the diagnostic are met. A diagnostic trouble code (DTC) is stored if the diagnostic is cancelled because the power consumption of the pump is varying excessively.
After reference phase the valve (2) in the leak diagnostic unit is activated and controls the air flow to the fuel tank to pressurize the fuel tank system. This change of air flow will cause the power consumption of the pump to fall briefly before the pressure builds up in the fuel tank system (3). A diagnostic trouble code is stored if the value for the power consumption of the pump drops too quickly, slowly or not at all.
Leak diagnostic, major leak (leakage greater than 1.0 mm)
The diagnostic for "major leaks" is carried out each time when the conditions for the diagnostic are met. The leak diagnostic unit pressurizes the fuel tank system, measures the power consumption of the pump (4) and compares this with a calculated desired value (B). A diagnostic trouble code (DTC) for a major leak is stored if the measured value does not reach the calculated desired value within a certain time (the time is determined by the fuel level in the tank).
Leak diagnostic, minor leak (leakage greater than 0.5 mm but less than 1.0 mm)
The diagnostic for minor leaks is run every other time that the conditions for the diagnostic are met. The diagnostic for major leaks is always run before the diagnostic for minor leaks. The leak diagnostic unit continues to pressurize the fuel tank system (5-6). After a certain amount of time (the time varies depending on the fuel level in the tank), the engine control module (ECM) checks the fuel tank system for leaks. This is determined from calculations based on:
- time
- the measured reference current consumption of the pump (A)
- the measured power consumption of the pump when the assessment is made
- the shape and character of the current curve during pressurization.
A diagnostic trouble code (DTC) is stored if a minor leak is detected.