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The 15 Mercedes-Benz GLK equipped with 274 engine has a history of 7.7W. Recently, the coolant temperature pointer has reached 120C while driving, and the temperature is too high to warn.
The car comes into the shop for repair:
The factors that lead to the excessive water temperature of the car: lack of antifreeze, fan failure, dirty water tank condenser, thermostat, water pump and other factors.
Check the ideas:
1. Antifreeze: whether it is missing (antifreeze liquid level), leakage, and whether to add unqualified antifreeze products.
2. Fan: check the cooling fan, whether the cooling fan is working properly, whether the cooling fan bearing is stuck or not, activate the cooling fan running at high and low speed, and whether it is working normally.
3. Water tank condenser: whether the surface is blocked by too much sundries.
4, thermostat: the operation of the thermostat needs to be involved with the rise of the temperature, and the coolant temperature increases gradually. when the temperature reaches (fixed value) about 97 ℃, the thermostat opens and touches the cooling pipeline between the thermostat and the radiator.
By observing the actual value of the coolant temperature sensor, it is found that the coolant temperature increases continuously with a range of 0.75 ℃. When the coolant temperature reaches 105.75 ℃, the cooling fan begins to run at a low speed. After waiting for a period of time, the coolant temperature decreases and the cooling fan stops running. After preliminary detection, no fault is found.

Inspection found that all the indicators are very normal, there should not be a high water temperature, in order to make the car owners feel more at ease to use, so we conducted a road test, here found the problem.
During the road test, it is found that the engine temperature will not be too high when the speed is above 60 km/h. But after driving at a speed below 60 km/h and turning on the air conditioner AC switch for a few minutes, the coolant temperature suddenly rose rapidly.
When the engine coolant temperature is too high, touch the cooling line between the thermostat and the radiator by hand, and find that the temperature of the pipe is not high at all, indicating that the thermostat is not turned on at this time. Try to use the fault detector to test the thermostat, and the result is normal. According to the working principle of the cooling system of the car and the analysis of maintenance experience, it is suspected that the coolant pump is not working properly.

Remove the coolant pump and check and find that the coolant pump is stuck.
The reason why the engine temperature is too high is that the rotary ball valve of the coolant pump is stuck, resulting in the coolant circulation being cut off. The rotary ball valve is controlled by vacuum, and the vacuum chamber keeps negative pressure to try to open the ball valve, which causes damage to the vacuum chamber for a long time.
For more detailed observation, I took a few more pictures.

A good comparison between the new pump and the old pump shows that the problem lies, and the vacuum-controlled small lever can not return to normal position at all.

When the vehicle is idling the engine in place, the coolant circulation required is small; while when the vehicle is driving at high speed, the heat dissipation is better, so it is not easy to have the fault phenomenon of excessive engine temperature. On the other hand, when driving on the urban road and turning on the air conditioner, the engine load is large and the heat dissipation is poor, the coolant can not get effective circulation heat dissipation, and there is the problem of high temperature of antifreeze after the pump breaks down.

This is how this pump works. This pump often occurs at 6-10W km, and the water temperature is sometimes high and sometimes low, so it is difficult to detect, but not to mention that the failure rate of this pump is really not low. If there is no problem in other places, there is a 90% chance that there is a problem here, and it is not repairable, so it can only be solved by replacing a new pump. A complete set of materials (water pumps, man-hours, antifreeze) is more than 2,000 yuan, which is not cheap. Needless to say, there is another Mercedes-Benz with high water temperature and the same type of engine. I hope this is not the problem.