An Iveco NJ2046 car equipped with SOFIM8142 diesel engine. Car owners reflect: after the engine starts, the idle speed of the engine has a slight jitter and similar "knock" sound, acceleration jitter and similar "knock" sound, and the engine speed can not be improved (less than 2000 rpm). The engine adopts the electronically controlled high pressure common rail system of BOSCH company. After testing with diagnostic instruments, the circuit part is normal. Since this fault occurs in the case of vehicle availability, it is possible to determine that the low-pressure oil circuit is free from failure, and the problem is that it is part of the high-pressure oil circuit. According to the phenomenon of engine jitter and similar "knocking" phenomenon in the fault phenomenon, the initial diagnosis is "lack of cylinder", that is, a few cylinders do not work malfunction. After the engine was started, the oil cut method was used to remove the engine. It was found that the third cylinder was not working properly. After flameout, the fuel injector of cylinder No. 3 was removed for inspection. It was found that there was corrosion and seizure in the injector. When the new injector is replaced, the engine is restarted. At this time the engine jitter and the "knock" sound are eliminated, but the speed of the engine is still within 2000 / min when the engine is accelerated, and the speed is still not improved and the fault indicator light is bright. Then the fault code is checked with the diagnostic instrument and the fault code is negative oil pressure deviation, which indicates that the actual common rail oil pressure produced by the high pressure pump is too large to be tracked by the theoretical oil pressure of the computer calibration, and the fault of the high pressure oil pump is diagnosed. Then the high pressure oil pump was removed and the oil pump was found to be seriously worn, and the actual supply flow was only 2/3 of the standard supply flow. At the same time, there was obvious rust trace in the pipeline when the high pressure pump was dismantled. Troubleshooting: after replacing the new high-pressure oil pump, restart the engine and speed up the inspection. At this time, the engine speed can be improved normally and the working conditions are normal. Fault analysis: from the troubleshooting process, the model actually has two faults, one is the engine jitter caused by the injector failure and the "knock" sound caused by the failure of the fuel injector. The main reason is that the fuel quality is not good and the injector is rusty and stagnant. Second, because of the serious wear and tear of the high-pressure oil pump, the actual supply flow is less than the standard supply flow, resulting in the engine speed can only be maintained below 2000 rpm. In the process of troubleshooting, the injector and the high pressure oil pump have water and corrosion phenomenon. After understanding, the engine does not have good fuel quality, so the high pressure oil pump is seriously worn out.