Leak diagnostics, original version (2005-)
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
- the EVAP canister purge 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 evaporative emission system (EVAP) valve
- the engine coolant temperature (ECT) sensor
- speed signal.
- 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 11.0-14.5 V. The voltage must be stable
- EVAP canister purge valve closed
- Low volume in the canister.
Fuel tank filler cap check
The exception from the above conditions is when the car has been refuelled. The engine control module (ECM) starts a check of the fuel tank filler cap after refuelling. This check is a simplified version of the leak diagnostic unit for major leaks. The fuel tank filler cap control is run while the vehicle is being driven. This allows the control module to check that the cap has been reinstalled. A diagnostic trouble code (DTC) is stored in the engine control module (ECM) and a text message is displayed in the driver information module (DIM) if the cap is missing.
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 diagnostics
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 a reference phase for leakage. During the reference phase (1-2) for leakage that is 0.5 mm, the pump in the leak diagnostic unit pumps 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 used 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 the reference phase (1-2), or if the value varies too much during the 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 several time in a row because the power consumption of the pump is varying excessively.
After the 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 atmospheric pressure and 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 that the fuel tank system for leaks. This is determined 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.