At the thermostat, it is necessary to check the temperature at which the main valve opens and its stroke. To do this, install the thermostat on the stand in a tank with technical glycerin. From below, rest the bracket of the indicator leg against the main valve.
Glycerin is heated from 78...80°C gradually, increasing the temperature by 1°C per minute with constant stirring, so that it is the same throughout the volume.
Take the temperature at which the valve opens by 0.1 mm as the temperature at which the main valve opens. For a serviceable thermostat, the main valve opening temperature should be 85...89°C, and the valve stroke should be at least 8 mm when the temperature rises to 102°C.
The simplest check of the thermostat can be carried out by touch directly on the car. After starting a cold engine with a working thermostat, the lower radiator pipe should heat up when the coolant temperature reaches 87... 92°C. In this case, the arrow of the coolant temperature gauge will not reach 3... 4 mm before the beginning of the red zone of the scale.