What’s Causing My Coolant Reservoir to Overflow?

Coolant, or antifreeze, is essential to regulating the temperature of your vehicle. It’s also extremely toxic and designed to stay inside a closed system.

If you’re seeing an overflow, it could be due to a radiator cap, thermostat, water pump, or radiator malfunction. Below are some things to check.



Be sure you have the proper level of coolant in your vehicle. Overfilling may cause overflowing.


Cheap and easy to replace, this is the first thing to check. Your problem may simply be a loose or damaged cap not keeping coolant where it needs to be. The radiator cap is designed to allow access to fluids in a ‘closed’ system. The coolant used to cool the engine does so at maximum efficiency when kept at specific pressure. Check your owner’s manual for proper cap replacement. The radiator cap comes in different pressures.


Most of the rest of these problems will cause a check engine light to turn on, but they could have an issue and not be noticed by your vehicle’s computer. A bad thermostat can improperly regulate fluid, and can lead to overflow.


How to tell if your over heating problem is caused by the water pump. Faulty water pumps slow or stop the flow of fluid through the cooling system. The result is overheating of the engine. You will see coolant fluid on the ground and around the water pump when the water pump has gone bad. Water pumps either work properly or go bad in a hurry. There is no “keeping an eye on it” for validation.


If the cap, thermostat and water pump are all fine, the reservoir is overflowing most likely because of a radiator problem. In older automobiles, the radiator was made of copper and aluminum. In more modern automobiles, the radiator contains a lot of plastic. The plastic radiator are light weight, but tend to ware out faster. Replacing the radiator is recommended over applying a patch if the leak is found.


If you’re seeing gas bubbles or some boiling in the liquid when you remove the radiator cap, a cracked head or block may be to blame. Do not remove a radiator cap from a hot engine to check. You can experience a shot of boiling water exploding under pressure. Your local repair shop can perform test to identify cylinder head or engine block cracks.

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