Thermostat 

The thermostat installed in your car has a crucial task to fulfill-it is the auto part responsible for controlling the amount of water-coolant mixture that is released into your engine system, all depending on whether your engine is running warm, cold or extremely hot. A stuck thermostat means that your engine will not receive the right amount of antifreeze and water each time it's needed. When this happens, the engine starts to heat up almost immediately, and overheating happens as a result of this.