The buildup of pressure caused by adding too much coolant to a car can cause the rupturing of hoses, internal pipes and even the radiator. When coolant is added to a car, it is cold, however, while the engine is running, the liquid starts to expand, causing a buildup of pressure. If the radiator is too full, this buildup of pressure is often too great for the internal workings of the car.
The excess pressure caused by adding too much coolant to a car usually affects the hoses first. This pressure causes the hoses to leak. It is recommended that car owners keep extra coolant in their cars at all times. This is especially important if the driver believes that the radiator is even slightly overfilled.
The pressure that builds up in a radiator is extremely strong. That is why car operators must never remove the radiator cap under the hood of a car after it has been running. The pressure in the system causes the cap to shoot off and the coolant to boil over. Usually, coolant is only added to a car in the overflow tank. This is a part of the system that is not pressured.