The coolant's boiling point is increased when under pressure, so this allows the coolant to get very hot but not boil. When it boils, it turns into gas, and this is not good for cooling as the gas is not a good heat conductor - it also dramatically increases in volumes which in turn increases the pressure in the system. The pressure relief valve built into the header tank cap will ensure that the hoses won't burst by releasing some of the gas, but this means lose of coolant which is allowed to escape.
If you run the engine with the header tank cap off, when the coolant warms up it will eventually boil - some of it will escape as vapour and some will simply gush out of the header tank so you will eventually have very little coolant left in the system which will lead tio further overheating...
You can drive with the cap off while watching the temp gauge, when it gets near the 80 degrees mark switch the engine off and let it cool down before driving off again.
However, this is also far from ideal - once you loose coolant, you will have air coming in and when there is air trapped in the cooling system, the gauge stops working correctly - the sender has to be in contact with the liquid coolant in order to measure the temperature, when it is in contact with an air bubble instead it will not register the temperature - so the gauge could show a cool engine while in fact it is boiling. Remember that the temp gauge only work properly when the cooling system is full on coolant...
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Topic: Coolant issues (Read 1496 times)