My car overheated and I was only able to put tap water in the overflow I noticed once I parked my car the overflow water inside started boiling. Is that because I needed to add 50/50 coolant. Do u think that would prevent it from boiling when it gets hot??

  • My car is doing the same...but the heatgage didnt show overheating....until yesterday.... when I got to garage radiator was empty and water bottle half from max..... I did check the morning and water was fine.... I checked everyday since this started.... last Sunday (week ago) I flushed the radiator to make sure there was no blockage..... awhile back someone worked on my car and did something wrong which cost me to replace the head .... after it started heating, I stopped let engin cooled down and drove, like that till the nearest garage..... after we cooled the car and filled the radiator I c
    – user15224
    Commented Feb 20, 2016 at 20:18
  • Hi Sherri, could you tell us what the car is and what engine it has? Mixing in 'coolant' won't really help it cool any better but it will prevent the water freezing in cold weather, which can crack your engine.
    – Sam
    Commented Mar 10, 2016 at 15:01

2 Answers 2


Adding more water than coolant will decrease the boiling point, yes. Tap water is also bad as it has many minerals in it, and can cause corrosion if too much is used.

However, the system is pressurized and should not boil unless the engine has reached around 260F degrees, under normal conditions. If your engine is getting that hot, additional damage may have been caused.

If the coolant is not actually boiling and what you see is simply bubbling in the overflow tank, you almost certainly have a blown head gasket. The bubbling is due to pressure from the combustion chamber being relieved into the cooling system through the leak in the head gasket. You should get it repaired as soon as possible, or further damage may result. You will continue to lose coolant, and the engine will not run as efficiently as normal.


'Coolant' from a dealership or accessory shop is 50% distilled water 50% antifreeze, as you may know. Using distilled(de-ionised) water removes any contaminants that would promote corrosion in your engines cooling sysytem over time, which gives the 'coolant' a longer service life. It has no effect on the boiling temperature of the coolant which remains at 100 degrees centigrade. Raising the system pressure by means of your radiator cap will raise the boiling point by the rate of +1 PSI = +1.5 degress centigrade. On some global hot regions, engines are run on distilled water without antifreeze. The over- heating you describe may be caused by a water leak in the system, a faulty thermostat, a faulty water pump, a blocked radiator, a collapsed hose, air in the system, or a faulty head gasket. You may need to have a repair shop look it over. Use coolant or 50-50 antifreeze mix to protect your ehgine from freezing in cold weather. A 50/50 mix will prevent freezing down to around -30 centigrade. Using either less, or more, then a 50/50 mix will increase the freezing temperature. ie 60AF/40W will freeze at -25C as will 40AF/60W. (You can't make a cup of tea on the top of Mt Everest as the lower atmospheric pressure boils water at 85C :-) I am English)

  • I think coolant increases the boiling point, yes? Though not sure if it matters since the whole system is pressurized...
    – rogerdpack
    Commented Aug 18, 2017 at 22:15

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