There are many reasons why your car is running hot or is outright overheating. Usually, the blame lies with your car's cooling system. For example, if the thermostat failed, it wouldn't be able to actuate and permit coolant to flow through and cool down your engine. Elsewhere, a bad water pump might also cause your car to heat up excessively. It functions to circulate coolant around the engine, so if the pump fails, no coolant is able to flow. Speaking of, contaminated coolant, low coolant levels, or even excess air bubbles in the coolant flow would disrupt its ability to absorb heat from the engine. Just as much, leaky or bad radiator hoses, a faulty radiator, or a radiator fan not working, would all impair the cooling system's ability to cool down a hot engine, too.