What Causes Car Radiator Overheating and Boiling

by Lina Schofield
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yellow car, a honda japanese sport car model image by alma_sacra from Fotolia.com

The radiator acts to protect the engine block from overheating when it is functioning properly. The radiator additionally keeps the pistons in working order, allowing the engine to run. Coolants pass through the engine block, flowing into the radiator to draw out excessive heat. The coolant then recycles through the engine. When the radiator fails to function correctly the coolant returning to the engine block is too hot, and as a result it boils over.


Whenever there is a damaged seal, excess air leaks into the radiator system, causing problems with the internal pressure levels. These pressure levels are extremely sensitive and directly affect the ability of the radiator to serve its cooling purpose. Air flow must be completely controlled, as well as the expansion of the coolant, otherwise the radiator boils over. Air also increases the rate of corrosion.

Head Gasket

A failed head gasket is usually diagnosed by the appearance of air bubbles in the coolant or foam in the expansion reservoir. A failed head gasket affects the ability of the water pump to circulate coolant through the system, and the result is overheating when the vehicle is in idle.

Radiator Cap

A radiator cap is composed of three independent seals known as the main, the pressure and the return seals. They act to control the pressure, expansion, and flow of the coolant throughout the radiator system. Improper circulation and overheating can occur without the essential function of these seals. It is recommended that you replace the cap every five years.

Radiator Fan

The fan helps keeps the radiator cool when the vehicle is not in motion. The fan is hooked up to the radiator through a network of various wires that can become prone to corrosion over time, and the radiator can overheat if the fan isn't providing adequate air circulation when at a stop or standstill if it gets hot enough.


The thermostat detects the levels of heat in the coolant and is designed to open when the coolant in the engine has gotten hot, letting the cooler coolant in the radiator flow into the engine and circulating the hot coolant back into the radiator to cool down. Problems with the thermostat that cause overheating mostly happen when it gets stuck in the shut position.


The coolant level and quality are important to monitor in the radiator. Fresh non-silicate, non-borate coolant is ideal, as silicate-based coolants tend to increase the risk of silt build-up in the radiator, and old coolant becomes acidic and destroys the internal radiator system. Low coolant level is usually a sign of a problem in one of the other parts of the radiator like the head gasket or radiator cap.

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