The mileage is about 200 thousand km, equipped with BMW 528i car with M52 electronic controlled engine, automatic transmission and automatic air conditioning system. User response: when the vehicle is running, the coolant thermometer of the dashboard is too high, and the function of air conditioning and refrigeration is invalid. After stopping, the coolant was ejected from the water inlet of the radiator tank. Troubleshooting: check the cooling system and find that the coolant in the sub tank has basically been lost. According to the maintenance records, the radiator, water pump and thermostat have been replaced in the previous maintenance work, so it is more likely that the cylinder pad tearing or the cooling fan is bad. Fill the auxiliary tank with water and use the water leakage tester to add the pressure of about 100kPa to the cooling system to check the sealing performance of the cooling system. Observe the change of pressure gauge pointer, there is no collapse phenomenon, indicating that the cooling system is not leaking. Because this test can not exclude the bad sealing factor of the tank cover itself, the seal of the tank cover is checked and the cooling condition of the cooling system is checked at the same time. Tighten the water tank cover and start the engine. The lotus root fan in front of the pump runs. There is an electric fan in front of the condenser. The fan is controlled by the engine control module and can operate in different gears through the voltage divider and the related relay. The lower left part of the radiator has a coolant temperature switch, pulling down the switch's 3 needle wire harness plug, short connecting the related terminals for testing, the electric fan can achieve high and low speed operation, proving that the fan itself is good. When the air-conditioner is turned on, when the air conditioning compressor is sucking, the electric fan can operate synchronously, indicating that the cooling command signal is good. Continue to observe the coolant temperature condition, about 5 min, the coolant thermometer pointer began to rise rapidly, more than 110 degrees Celsius. Hand touch the upper and lower pipes of the radiator, the water pipes above are hot, and the pipes below are warm, indicating that the large circulation circuit of the cooling system has not been opened yet. This situation usually has two reasons, one is the water pump damage or poor performance, the coolant can not be reflux; two is the damage of the thermostat, blocking the radiator pipe. The pump was removed and examined. It was found that the impeller of the pump was plastic and pressed into metal shaft. No signs of cracks and looseness were found in the inspection, intact. Remove the thermostat and test it in hot water. The thermostat valve has been closed. Observing the appearance of the thermostat carefully, it is found that there are several cracks at the end of the paraffin metal package. It is estimated that the internal wax has been leaked and the thermostat opening function has been lost. Before the installation of the new thermostat, the appearance of the new thermostat has a large diameter, the size of the paraffin metal package is larger, and the sealing effect is good, which proves that the old parts do have a quality problem. Replace the thermostat, add coolant and start the engine, and the cooling system can carry out the large circulation backflow. The coolant overflow phenomenon disappeared when the road test was carried out, but the coolant temperature was slightly higher from the indication of the coolant thermometer. For this problem, the author thinks that the gas resistance in the cooling system piping is caused by it. The design of the cooling system piping of BMW automobile is very special. Every time after emptying, the injection of coolant must be carried out by standard exhaust operation. There are usually two vent holes on the coolant line, one located on the radiator upper pipe and the other on the auxiliary water tank. Before the injection of the coolant, the two vent plug should be unscrewed first, and the ignition switch (the engine is not running) is opened to open the air conditioning system so that the auxiliary electric pump of the air conditioning system is operated. In this way, during the filling process, the coolant is in a flowing state, and the air in the pipeline can be discharged quickly and thoroughly. When the cooling liquid is added to the calibrated height of the auxiliary tank, tighten the vent plug, start the engine, make several acceleration movements in situ, and observe the flow state at the water injection port, at the same time, the reflux hole inside the auxiliary tank shall be seen, the coolant is reflued to the auxiliary tank, and the bubble is not brought out, and then the lid of the water tank can be tightened. . According to the above method, the temperature of the coolant is higher and the air conditioning system returns to normal. The high temperature of the coolant is caused by poor quality thermostat. The coolant can not carry out the big cycle reflux. The temperature is too high and it is boiling from the cover of the vice tank. The high temperature of coolant affects the air conditioning system and the refrigeration function fails. There are two points to be summed up in this case: one is not to believe that the new component is good, after the occurrence of the related failure, it should be carried out in accordance with the basic maintenance methods, to find out the problems and eliminate the failure; two is to pay attention to the structural features of the BMW model, some BMW cars are equipped with auxiliary tanks, and there is no floating device inside. Therefore, the density of coolant will affect the signal of water level sensor. If the coolant concentration is too low, even if the injection is applied to the required height, the water level alarm information will appear on the dashboard. Therefore, the original coolant must be selected and charged according to the correct ratio.