Coolant Recovery Tank
A coolant recovery tank
The coolant recovery tank, also called an overflow reservoir, is a tank that allows the coolant to expand without being forced out of the radiator. The tank can also ensure that the radiator is always full as it allows the radiator to be refilled as the engine cools.
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As the engine heats up, the coolant expands as its temperature increases. The increase in heat is accompanied by an increase in pressure. The radiator pressure cap, which uses a spring to maintain a seal on the radiator fill hole. Once the pressure reaches around 15 psi, the force of the spring is overcome, and the cap rises.
As the cap rises, the opening of a tube is exposed. The coolant under pressure is forced through the tube into the recovery tank.
As the engine cools, the process works in reverse. Instead of pressure forcing coolant out of the radiator, a vacuum draws it back in. This keeps the coolant at the proper level throughout various engine temperatures.
Measuring Coolant Level
The coolant level can be measured by a dipstick in the recovery tank. The dipstick has a range of hot and cold. The coolant level should be at the "cold" mark when the engine is cold, and hot when the engine has reached running temperature.
Coolant recovery tank dipstick. Note the Full Hot and Full Cold markings
In the event that too much coolant is in the system, the recovery tank itself has a pressure cap that will allow the excess coolant to exit the vehicle. If the pressure builds too much inside the radiator and recovery tank, the expanding coolant will force the cap open enough for the excess pressure to be relieved.