We start a cold engine and control the temperature of the coolant according to the temperature gauge on the instrument panel. Temperature of the beginning of opening of the main valve of the thermostat - 85+2°С (it's on the thermostat cover). As long as the engine temperature is below the specified temperature, the main thermostat valve must be closed. In this case, the liquid will circulate in a small circle. This can be determined by touch - the lower radiator hose and the radiator itself should be noticeably colder than the thermostat housing, through which hot coolant circulates.
When the coolant temperature reaches 85°C, the main thermostat valve begins to open and a gradually increasing flow of hot liquid enters the radiator. In this case, first the radiator itself, and then the lower radiator hose will begin to heat up.
At a temperature of 102°C, the main thermostat valve is fully open and the main fluid flow passes through the radiator. In this case, the radiator should be hot in the upper zone and slightly colder in the lower zone.