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The mileage is about 40 thousand km, and the model is E84, the BMW X1 sport utility vehicle equipped with N46 engine in 2013. User response: when the vehicle is running at high speed, it will alarm the water temperature. Inspection and analysis: a preliminary inspection of the maintenance personnel found that the car has a large number of seepage traces around the expansive tank of the coolant, and the cooling fluid is seriously short for a long time to suspect the leakage of the cooling system, so the system is tested under pressure, and the results are found to have good sealing performance. Detection of engine control unit failed to identify fault records with high water temperature. The operation of the cooling fan is controlled by the fault diagnosis instrument to ensure that the fan works properly. N46 engine pump is mechanical, so long as the engine works, it will work, so the probability of failure caused by the pump is very small. He decided to focus on the thermostat opening, cooling system circulation and cooling system air resistance. In view of the Limited mileage of the vehicle, it is first suspected that there is air resistance in the cooling system. Add and exhaust the coolant according to the standard process, and adjust the liquid level to the standard position before commissioning. Soon the pilot returned, reflecting the high temperature of the water and found that the pipe connected to the thermostat was cool. It is possible to judge that the thermostat is not opened. According to the circuit diagram, the thermostat is directly controlled by the engine control unit. Comparing with the normal vehicle, it is found that the waveform of the control signal is the same as that of the normal vehicle, which indicates that the vehicle is no problem in control. It can be more affirmatively stated that the thermostat is broken, so the thermostat is replaced. Originally thought that the fault has been ruled out, but the test result is still a failure. Is it a radiator? After this doubt, he immediately touched the radiator just after the car was finished, and found that the temperature difference between the top and bottom was very large. This shows that the coolant did not flow through it. Is it really a radiator blockage? So replace the radiator after commissioning, the fault still exists. So far, all the parts that could be thought of have been replaced, leaving only the pumps that were originally considered to be the least likely to fail. Calm down and think about it. There is a temperature difference between the radiator and the radiator indicating that the coolant has not flowed. The radiator pipe to the thermostat is cool and proves that the coolant does not flow. The control of the engine control unit is no problem. Why does the coolant do not flow? The coolant must be driven by the pump if it wants to flow. After the failure of the replacement parts above, the problem is concentrated on the pump. Demolition of the water pump hand rotation, found that some pumps issued cards, the original problem is here. Troubleshooting: replacement of pump test run, vehicle completely restored to normal.