Damaged thermostate
The thermostat is a simple element of the cooling system, but due to a malfunction of the function, it causes big problems.
Damaged in the closed position, it practically does not allow driving, and in the open position all the time, it significantly reduces driving comfort.
It is impossible not to notice the damage to the thermostat in the closed position. The first sign is that the temperature gauge moves into the red area very quickly. If the driver ignores this signal, clouds of smoke will soon appear and the engine will seize.
Overheating can occur after a few kilometers. Diagnosing this problem is simple. If the fluid level is normal, the water pump drive V-belt is tensioned correctly, the radiator temperature is not too high, and the sensor shows a high temperature, then 
The thermostat is to blame for this condition. Under no circumstances should the radiator cap be unscrewed when the engine temperature is high, as this will cause sudden release of liquid or vapor, which can cause severe burns.
The thermostat can also be damaged in the open position. Such a malfunction is much less dangerous for the engine, since there is a large circulation of fluid all the time and the engine is not in danger of overheating. However, prolonged underheating also adversely affects the operation of the engine. In summer, such a defect is almost imperceptible, and in autumn and winter it is clearly visible. This significantly reduces driving comfort, as the heating hardly works. An engine running all the time on a high cycle, i.e. with full use of the cooler at low temperatures, will systematically underheat. This leads to a significant increase in fuel consumption.
This fault is also very easy to diagnose. One method is to observe the temperature index. If the engine reaches operating temperature in a parking lot or in a traffic jam, and while driving it drops to a minimum, this indicates damage to the thermostat. You can also check 
when the engine warms up, the temperature of the rubber hoses that supply fluid to the radiator. If they are both at the same temperature, the thermostat is definitely defective.
Carry out additional tests, i.e. remove the thermostat and check in hot water if it opens, it is not worth it. This is a waste of time and money.
The thermostat for standard models costs from PLN 20 to 50, so if you suspect that it is not working properly, it should be replaced. When replacing the thermostat, you should also decide to replace the coolant. When disassembling the thermostat, some of the liquid will still leak out, so instead of topping up, it is better to replace all the liquid on this occasion. We will save some money by doing these steps in one go.
