First it is important to determine whether your coolant is boiling or your experiencing bubbling from the overflow tank
Boiling Coolant indicates and over heating engine.
A vehicles system is pressurized and should not boil unless the engine has reached about 260F degress (under normal conditions)
Boiling coolant can be the result of a faulty or wrong PSI cap. Failed radiator, and or even contaminated fluids used in place of prescribed ant-freeze. Note, the use of tap water will lower a vehicles boiling point and the minerals can be damaging to the entire cooling system.
Bubbling coolant indicates a blown head gasket.
If the coolant is bubbling in the overflow, it is imperative that you diagnose and treat what is most likely a blown head gasket immediately. The bubbling is the result of releasing pressure from the combustion chamber into the cooling system through the head gasket. It is imperative that you diagnose and fix a blown head gasket immediately. The vehicle will continue to lose fluid and more severe damages may also then occur.
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