Thermostats are used to manage engine temperature, not only for cooling purposes but also for proper engine operation. A car will run and can be driven without a thermostat, but doing so will cause a number of problems.
Modern automobile engines are precise instruments designed to operate within a specific temperature range for peak performance. The thermostat is used to regulate the engine to keep the temperature in this desired range. Without a thermostat, the engine will often run too cool, which will reduce performance.
The windshield defroster uses heat from the engine coolant to clear the windshield. Without a thermostat to block the flow of engine coolant, the engine coolant will take much longer to warm up. This will increase the amount of time needed to clear the windshield.
Driving without a thermostat will also reduce passenger comfort in cold weather. The missing thermostat will increase the amount of time needed to heat the coolant and keep the coolant at a lower temperature. As heat from the engine coolant is used to operate the heater, the passenger cabin will also receive less heat.
