Vehicle information
A 2010 BYD F0 sedan with a mileage of about 110,000km and equipped with a D371 QA engine.
Fault phenomenon
Fuel consumption has risen from 5L to 8L per 100km.
Diagnostic analysis
The repairer consulted the data and learned that the vehicle uses a 3-cylinder engine with a displacement of 1.0 L. Because of the lightweight body, the rated fuel consumption is very low. When the engine is idling, observe the data flow and find that the target idle speed and short-term injection correction are high, and other parameters are normal.
For a 3-cylinder engine, a target idle speed of 920 r/min is within reason to maintain smooth running. But the car’s problem is high fuel consumption, so even this subtle anomaly must be taken into account.
Since the idle speed is actively increased by the engine control unit, there must be some sort of fault that the control unit can detect. It was observed that the idle load rate of the engine was not high, so there was no reason to increase the idle speed in response to a change in load then there was only the problem of a large short-term injection correction.
High injection correction is a problem with the injection control system itself. In this case, the mixture concentration is not guaranteed, and to avoid combustion abnormalities, increasing the idle speed is an effective measure. It seems that the problem of this vehicle is centered on the short-term injection correction amount.
Observing the front and rear oxygen sensor signals, it was found that the rear oxygen sensor signal always stayed at 500 mV, which is abnormal. This indicates that the rear oxygen sensor has lost its role in providing feedback correction for fuel injection. In this way, the problem gradually became clear. The rear oxygen sensor signal is abnormal, the injection correction increases, the total speed increases, these phenomena are necessarily related. The finger is pointed at the rear oxygen sensor.
Fault resolution
Replacing the rear oxygen sensor, a test drive revealed that the rear oxygen sensor signal began to change, the amount of injection correction decreased, and the target idle speed dropped to 860 r/min (Figure 2). A week after the delivery of the vehicle returned to the user, learned that the fuel consumption completely returned to normal.
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