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The BMW 528Li (F18) car with a mileage of about 116 thousand km. Customer response: when the car starts up, it will shake at idle speed and run like a broken oil at high speed. Fault diagnosis: Car confirmation, after the start of the vehicle running normally, the diagnosis instrument diagnosis without any fault code exists, manual addition of the detection plan has no results, again ask the customer fault phenomenon, customer said when the vehicle cold car 5min and above, restart the discovery of fault phenomenon. The specific phenomenon is: at first, the idle speed is normal, but after about 5S, idle speed begins to decrease and jitter. According to the conventional way of thinking, we should first make clear whether the jitter caused by a single cylinder is not working well or the jitter caused by other factors. Try to diagnose with a diagnostic instrument. After reading the data stream after idling down, no cylinder is found to work well, and the influence of a single cylinder is excluded. Next, we need to find other factors that may cause engine jitter, first of all, doubt the idle jitter caused by the unstable oil pressure. After joining the oil pressure gauge, according to the customer's meaning, many cold cars start up and found a regular phenomenon. The oil pressure will keep the pressure of 500kPa at the start of the start (the pressure required for the engine working normally), and the oil pressure will decrease after about 5S, when the oil pressure drops to the 300kPa (sometimes it can be reduced to 200kPa), with the engine speed down and slight jitter. It is confirmed that engine idling caused by abnormal oil pressure. Because of the manual addition of the detection plan, the vehicle needs to be programmed, so the explanation for the customer at that time was due to the instability of the oil pressure caused by the program problem, which led to the engine shaking at idle speed and after starting. The client finally agreed to leave the car programming, and there was no improvement in the barrier phenomenon. Why does the oil pressure drop? EKPS? Fuel pump? The plug connection?... Suddenly a series of things appeared in front of the eyes. Of course, we should check from the simplest and check that the plug of the oil pump has no obvious ablation and looseness. Since no suitable adapter was found at the end of the oil pump, the adapter was attached to the EKPS plug to measure the power supply of the fuel pump. It is found that the oil pump voltage will be kept near the vehicle voltage when starting, and the power supply voltage of the oil pump drops to 9V, sometimes 6V (explanation: this engine's electric fuel pump (EKP) is manipulated by the PWM signal by the EKP control unit. The EKP control unit receives the corresponding request of the digital engine electronic system. The request includes load and speed information before, and now it is regulated according to pressure, so the voltage is slightly lower than the vehicle voltage, and then the voltage will remain on the vehicle voltage. Since no similar observations have been carried out before, the same type of 530Li (with F18 and N52 engine) is compared, and the side voltage of the view side is on the vehicle voltage, and then it is kept at about 10V. Suspect that EKPS has problems, and after adjusting it, it is found that the failure phenomenon has not changed, indicating that it is not caused by EKPS. It is doubted that the power supply of the fuel pump is affected by the unstable power supply of the vehicle, and the power supply voltage of the EKPS is measured at the start of the EKPS, and it is found that the power supply voltage is stable and there is no fluctuation. Check the fuse is not abnormal, contact no ablation loose phenomenon. There is no way to explain why the voltage will drop, and then the normal phenomenon. In the end, it is suspected that it is caused by electromagnetic interference. 6 ignition coils were seconded from the spare parts department, and the test found that the fault phenomenon still existed. Attempts to clean the throttle and other measures are still fruitless. It is doubted that the oil pump is defective, but the following problems such as refueling and trial run are troublesome after the pump is switched. After that, the oil pump is working, and it should not lower the power supply voltage of EKPS to the oil pump. Because there are more vehicles in the workshop, the car is put aside for other cars. Finally, the customers are in a hurry and can not add fuel pumps. I am not sure whether the fuel pump is causing problems. Because I am not sure that the root cause of the failure is on the fuel pump, so there was no bottom at that time. There was an episode after consultation with the maintenance receptionist: at that time, the receptionist said, "are you sure it was caused by the fuel pump? If the change is bad, the customer will not pay. This sentence gives me a monk, and I dare not order the oil pump. Later to find the supervisor, the director said: "no matter, fuel pump is not the C0 part of the warehouse index, meaning" death ", is the bad sale of goods, the impact of the warehouse assessment index), the bad change to the part, no money. With the sentence in charge of the sentence in the heart of the bottom, anyway, the back road all think well, and finally ordered a fuel pump, because the customers come in again when I do not work, so my colleague helped me to replace the fuel pump, after the test, the test is normal. Troubleshooting: update the electric fuel pump. After the refueling trial run, the fault will disappear. Fault summary: to the end, it can be found that the ultimate killer of the fault is caused by poor fuel pump work. Later, it might be possible to find out if the measurement of the working current of the fuel pump could be found. And the quality of the fuel pump will lead to voltage changes, that is to say, the load will affect the voltage.