Leak Diagnostic
Leak diagnostic
General
Vapor from the fuel in the fuel tank is routed to and stored in the EVAP canister. It is then guided into the combustion process via the canister purge (CP) valve and the negative pressure in the intake manifold. A leakage diagnostic has been introduced to ensure that the fuel tank system is not leaking. This diagnostic is designed so that the system is able to detect a leak / hole 0.5 mm or larger. Diagnostics are also carried out on the EVAP system.
The fuel tank system consists of the fuel tank, fuel filler pipe, roll-over valve, EVAP canister, canister purge (CP) valve and all cables between these components. In order to perform diagnostics, the fuel tank system is equipped with a leak diagnostic pump. The leak diagnostic pump is vacuum driven and pressurizes the fuel tank system. The vacuum that operates the leak diagnostic pump comes from the intake manifold via a non-return valve. The engine control module (ECM) activates / deactivates a three-way valve in the leak diagnostic pump (vacuum / atmospheric pressure). The pump action is obtained using a spring loaded diaphragm.
A position sensor is mounted on the diaphragm. This allows the control module to determine when the three-way valve should be activated. The position sensor acts on a contact breaker. The contact breaker transmits a high or low signal to the engine control module (ECM). The control module activates the three-way valve when the diaphragm is in the resting position. A vacuum is generated. The diaphragm is switched to the active position. The three-way valve is deactivated when the control module determines that the diaphragm has reached the activated position. The atmospheric pressure is allowed to enter and the diaphragm returns to the resting position.
In order to detect leakage in the fuel tank system, the leak diagnostic pump is used according to the description in "Leak diagnostic" below.
The diagnostic takes place once per operating cycle if the following criterion are met:
- The battery voltage must be correct
- The engine coolant temperature (ECT) and intake air temperature (IAT) must be 4-60°C
- The engine coolant temperature (ECT) must have dropped by at least 15°C since the previous operating cycle
- The atmospheric pressure must remain constant.
Function test
The diagnostic is divided into different phases in which the components involved are tested before the actual leak diagnostic begins. The diagnostic is cancelled and a diagnostic trouble code (DTC) is stored if a fault is detected in any of the phases. The diagnostic test works as follows:
- The system must be depressurized. The control module checks that the pump diaphragm is in the resting position. Diagnostic trouble code (DTC) ECM-65, faulty signal, is stored if the control module registers that the position is active
- When the engine is started, a vacuum is generated in the intake manifold. The three-way valve opens a connection to the intake manifold via the non-return valve. The diaphragm is moved to the activated position by the vacuum. Diagnostic trouble code (DTC) ECM-65, high signal, is stored if the control module registers that the diaphragm is still in the resting position
- The three-way valve closes the connection with the intake manifold. The non-return valve closes. A connection to the atmospheric pressure opens. The spring-loaded diaphragm must then return to resting position. Diagnostic trouble code (DTC) ECM 65, low signal, is stored if the diaphragm does not return to the resting position.
Leak diagnostic
- The leak diagnostic pump is pulsed (A) rapidly at a rapid frequency. This is to pressurize the fuel tank system to the desired pressure (C)
- This rapid pulsing is interrupted regularly (B). This is to check that the correct pressure in the fuel tank system is reached. This is done by gauging the time taken for the pump to switch from active to resting position using the switch in the leak diagnostic pump. The constant pulse (A) and pressure check (B) alternate until the desired pressure (C) in the fuel tank system is obtained. If vehicle speed exceeds 40 km/h, there is no pressure check (B). The pump is activated and deactivated a fixed number of times instead. If the pressure (C) is not reached, the control module interprets it as a leakage. Diagnostic trouble code (DTC) ECM-6A, fuel tank filler cap missing, is stored
- When the pressure in the fuel tank is detected by the control module using the leak diagnostic pump, the control module begins to check for smaller leaks (D). To do this, the control module measures the time the diaphragm takes from active to resting position. Diagnostic trouble code (DTC) ECM-68, large leak, or ECM-68, small leak, is stored if this time is too short
- If no leakage is detected in the fuel tank system, the flow in the evaporative emission (EVAP) system is checked for blockage. This is done by pulsing both the canister purge (CP) valve and the leak diagnostic pump at the same time. The control module checks that the pressure in the fuel tank drops (E). If the pressure does not drop, the control module interprets it as a blockage in the evaporative emission (EVAP) system. Diagnostic trouble code (DTC) ECM-6A is stored. The canister purge (CP) valve is pulsed until the fuel tank system is depressurized. The diagnostic takes place in the next operating cycle.