A BMW 530 car with a distance of about 130 thousand km. The car has a low level of alarm in the liquid level of the coolant. Fault diagnosis: first test run after receiving the vehicle, confirm that the fault is true. Check cooling system found coolant compensation tank cracking, other components are not abnormal. Replace coolant compensation tank, add proper amount of coolant, test run, coolant level is normal. Soon after the customer left, he called the repair shop to report that the coolant temperature was too high. When the trailer enters the factory, check the coolant level. The customer reflects the normal running of the vehicle. The cooling liquid temperature is too high after the original idling. Use fault detector to read fault information as "thermostat control, open circuit". Check the connection of the thermostat control harness, normal. After changing the thermostat, test the car, everything is normal. However, after the customer raised the car 2h, it again reflected the alarm of the vehicle coolant temperature too high. After re entering the plant, the liquid level of the coolant is normal. The fault information is read by the fault detector, which is "self diagnosis of cooling fan, hardware failure", "coolant pump and speed deviation". Check that the cooling fan's power supply and grounding are normal. The cooling fan is activated by the diagnostic system, and the cooling fan works normally. Check the power supply and iron lap of the coolant pump, normal, the related line is normal and the connection is reliable, the coolant pump works well. In the road test, the coolant temperature is maintained at 95 C. After the engine idle speed is 20 min, the coolant temperature is found to rise to 119 C, and then the coolant temperature is too high to alarm. The inspection found that the cooling fan was not running at the moment. After pressing the air conditioning refrigeration switch, it was found that the cooling fan still did not turn, and the air conditioning refrigeration effect was poor. It is doubted that the cooling fan and the coolant pump have trouble, changing the cooling fan and the coolant pump to test the car, the vehicle runs all normal, the engine idle speed is about 20 min and the cooling liquid temperature does not alarm, but it is found that the cooling liquid temperature is maintained at 104 C at idle speed, and it is generally not super to check the temperature of the coolant when the same model normal car is idle. 90 degrees centigrade. Why is the coolant temperature still high when the engine is idle? It is found that there are a lot of dead tree leaves in the interlayer between the air-conditioner condenser and the radiator of the car (Figure 1). Most of the clearance of the heat sink has been blocked. After the dead tree leaves are removed, the engine is running for a period of idle speed, and the temperature of the coolant is maintained at about 90.