Problems That Cause a Car Engine to Run Hot

by John S. Kepler

Automobile engines that burn fuel require cooling. Generally, this cooling is managed using a liquid-anti-freeze mix that is pumped through a radiator. There are a number of things that can go wrong with an automotive cooling system that will cause it to run hot.

Low Coolant

If there is a leak in the system that allows coolant to escape, there will not be enough coolant to absorb the excess engine heat. Coolant loss can occur through holes in the radiator or radiator hoses, a faulty radiator cap, a leaking water pump, damaged heater core or a blown head gasket. Improper water-coolant mix can also contribute to coolant loss.

Fan Problems

The fan draws air through the radiator when the car is at rest, or moving slowly. Some fans are driven by a belt and pulley system. If the belt breaks, comes off or becomes too loose, the fan, and in many cases the water pump as well, will stop spinning. This will cause the engine to overheat very quickly. Engine driven fans generally also have clutches that cause the fan to idle when the engine is cool, and spin only when the engine is warm. A faulty fan clutch can also cause an engine to run hot. Some cars also have electric fans. Problems with the fan motor, or the thermostat that turns the fan on and off can result in overheating.

Radiator Problems

The radiator must be clean and free of obstruction inside and out for the system to work properly. During the lifetime of a vehicle, scale and deposits can build up on the inside of the radiator, and block or reduce the flow of coolant. Alternately, blockage of the radiator cooling vanes will limit airflow through the radiator. This can be caused by bent vanes or obstructions. Even a heavy build up of dead insects can result in decreased air flow and high engine temperatures. Clogging, either internally or externally, will cause an engine to run hot.


The thermostat controls the waterflow through the engine. It is not uncommon for this device to fail in cars that are a few years old. A thermostat stuck in the closed position, or that does not open fully, will cause rapid heat buildup.

Engine Problems

Not every overheating problem is directly related to the cooling system. A blown head gasket can force exhaust gasses into the coolant, and cause it to heat up. Likewise, a faulty oil pump or low oil level can result in increased motor friction and heating. Less commonly, overly-advanced ignition timing or a lean fuel-air ratio can lead to knock, which will result in a hot engine.

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