what causes the water in the radiator water tank to over boil?

Hi Johnathon,

There could be a number of components of the cooling system that could be the problem, but the main result is usually overheating. This will cause the water/antifreeze mixture to boil or if something blocks the flow then it will expand.

Likely causes include:

1) Failed/stuck thermostat - This is a biased metal inline with the waterflow and is usually housed just outside the engine at the front. Usually the housing is where the top radiator hose connects to. The thermostat will normally be closed when cool (under approx. 190 degrees F). When the temperature reaches the thermostats rated value then it will "pop" open (the bias metal bends) allowing the hot water to flow out of the engine into the radiator. When enough water flows through the radiator and back into the engine to cool the water below the rated value, then it will close.
Now if the thermostat fails in the closed position, then the engine will get too hot.

2) Clogged/leaking radiator - The radiator is where water is circulated through when the engine reaches a certain temperature and needs to be cooled. After the thermostat opens the water is pumped into the radiator which is basically alot of tubes that the water goes through. When air is forced passed the tubes (by either traveling down the road or a fan) the heat is removed. The water continues its circulation until it reenters the engine via the lower radiator hose.
If the radiator is clogged or partially clogged then the water cannot circulate and cool down for return to the engine, thus overheating.
If leaking, usually you can smell a unusual smell when the system is running hot. Look for white/greenish stains along the sides and underneath the radiator.

3) Water pump - The water pump is usually an impeller (mechanical fin-based motor that moves liquids when turned) that is turned by the engine by use of a belt.
It forces water to cirulate throughout the engine and also to the radiator when the thermostat is opened.
The water pump can fail and water will either not be pumped at all or dramatically reduced.
Most common failure is either lockup due to bearings failing or broken impeller fins.
Most automobiles have a "weep hole" near/underneath the water pump where coolant will drip or run out depending on the failure. Strange smells and small pools of greenish fluids under the engine could be a sign of a failing water pump.

4) Cooling fans - The cooling fans generally are used to assist in forcing air over the radiator to cool the water/antifreeze mixture. When the automobile is not moving or slowly moving due to traffic, then air is not moved across the radiator.
Some automobiles cooling fans are driven by belts and some are electrical.
If a belt is slipping or an electrical sensor has failed, the fans my not be cooling and forcing enough air over the radiator. Most electric fans can be checked by turning on either the air conditioning or defrost as this should turn on the fan.

5) Low coolant or improper mixture - Water's approximately boiling temperature is 212 degrees farenheit and automobile engines can get hotter than that. The way to lower its boiling temperature is to mix it with a chemical that has a higher boiling temperature (antifreeze is the common term). Usually a 50/50 mixture of water and antifreeze will allow much higher boiling temperatures and also has chemicals to help prevent rust and corrosion in your radiator.
If an automobile is low on coolant (most likely due to a leak or failing water pump weeping) then there is not sufficient coolant to transfer the heat from the engine and cause the coolant mixture to reach its higher boiling point.
If your system has been filled with water only or an improper mixture then the boiling temperature could be reached too easily.

Hope these items answered your question and anyone else who wants to know.

KP

Kenneth P.
December 2006