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 .
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.