Radiator Boils Over when the Engine is Semi-hot
The radiator boils over even though the engine is semi-hot. The two possible reason is causing this problem, first is a leaking radiator cap, second is a wrong anti-freeze/water ratio.
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Possible Cause why the Radiator boils over even when the Engine is semi-hot
Leaking Radiator Cap
If the radiator boils over even when the engine is semi-hot, it is most likely cause by a leaking radiator cap. The radiator cap is the one that keeps the entire engine cooling system under pressure which also resulted to rising of the boiling point of the coolant inside the radiator. Now, if the radiator cap is leaking the boiling point of the coolant will be less than that of the normal. If this happens the engine and the coolant will only need to be semi-hot for the radiator to boils over.
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Remove the radiator cap, but before removing make sure that the engine cools down. Examine the radiator cap for any cracks on the rubber part then put back the radiator cap and check if the cap is securely attached on the radiator fill hole. If there is something unusual on the radiator cap like looseness then replace the cap with a new one. Usually this will fix the boils over of the radiator even when the engine is semi-hot.
Wrong Anti-freeze/Water Ratio
Anti-freeze is the one that keeps the water from freezing with added into the water when the temperature of the outside surrounding is very low. The anti-freeze also raises the boiling point of the water usually the water coolant ratio is 50/50 mix. With this mixing it is adequate for both freeze and boils over protection.
Now if the mixing is not correct like there is more water than anti-freeze then the coolant will boil at much lower temperature in this case even when the engine is semi-hot.
Have the entire cooling system back flushed to completely remove the old coolant inside the cooling system then replace the coolant with a 50/50 mix of anti-freeze and water.