Why Does My Car Overheat When Idling?

by Paul Novak
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The cooling system of an automobile is a balanced combination of components working together to efficiently remove the heat from the engine and dissipate it into the air. When one component fails to operate properly, the end result is the same: overheating, aggravation, and sometimes expensive repairs. If your vehicle stays cool when driving, yet overheats when it comes to a rest, there are several likely reasons.

Fan Clutch

Engine cooling is accomplished by moving air through the radiator, which removes heat from the coolant before it is returned to the engine. When a vehicle is at speed, air is forced through the radiator through the motion of the vehicle. When a vehicle is stopped it relies on a fan to draw air through the radiator. On vehicles with a mechanically driven fan, the fan speed is controlled by a fan clutch. When this clutch becomes worn, the fan does not rotate at the proper speed and is unable to draw a sufficient amount of air through the radiator. This results in higher coolant temperatures.


On vehicles with an electrically powered fan, the fan is activated by a temperature sensor. When the vehicle is stopped air is no longer forced through the radiator by the motion of the vehicle, causing coolant temperature to rise. When the temperature exceeds a certain level, the sensor activates the electric fan which draws air through the radiator and cools the engine until the temperature is sufficiently lowered. A faulty sensor will not turn on the fan when the vehicle is stopped, causing the engine to overheat.

Fan Shroud

The fan shroud directs the air pulled through the radiator by the fan. If the shroud is missing or is damaged, air can escape around the shroud, reducing the amount of air pulled through the radiator by the fan. The result is less heat dissipated into the air by the radiator, causing the engine to overheat, particularly when the car is not moving.

Obstructed Radiator

Dirt, bugs, and road grime can clog the closely spaced fins of a radiator.If the airflow through the radiator is obstructed, too little heat is dissipated, resulting in overheating. When the vehicle is at speed, a sufficient volume of air is forced through the radiator, but when the vehicle is stopped the fan cannot draw enough air past the clogged radiator fins to carry away the heat from the coolant, resulting in overheating.

Internal Clogging

If the radiator has internally clogged cooling tubes, the flow of coolant through the radiator is impeded and slowed down. At higher engine speeds, the coolant flows through the radiator faster, overcoming the clogs. When the vehicle is stopped, engine speed is reduced, reducing the flow of the coolant which can no longer overcome the clogging, and overheating results.

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