1.6.4 Near empty fuel tank functionality

A near empty fuel tank can cause air bubbles in the fuel delivery system and thus causes a lean A/F ratio. First the lambda controller and then the fuel trim adaptation tries to compensate. This can cause erroneous MIL illumination, because the fuel trim adaptation factor goes over a diagnostic threshold.

The following flowchart shows the functionality to ensure MIL illumination if the fault is correct and to avoid erroneous MIL illumination in case of a near empty fuel tank.

• If one of the adaptation factors exceeds its diagnostic threshold, and the fuel tank is detected as being near empty, setting of the error is delayed for a certain time (depends on engine intake air mass). Simultaneously, integration of the engine intake air mass begins.
• If the integrated engine intake air mass (as a measure for fuel consumption) exceeds a calibrated threshold and the fuel trim adaptation factor still exceeds its threshold, the result of the fuel system monitoring is valid and the fault is set.
• If the adaptation factor decreases under its diagnostic threshold before the integrated engine intake air mass exceeds its threshold, there is no fault detection. The integrated engine intake air mass value is stored for later refueling detection.
• Refueling detection is started, as soon as the adaptation factor returned into its valid range. Refueling is detected, if the intake air mass, that is then integrated, exceeds a calibrated threshold (current integrated value minus stored value exceeds threshold).