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Coolant expands as it becomes warmer. Modern cars have sealed coolant systems so that it is completely full of coolant and the expanding coolant spills into the expansion tank. When it cools, a vacuum breaker valve in the valve cap allows the fluid (or air) to return to the cooling system. Coolant expansion tanks are typically translucent to show their coolant levels and are marked to show the full hot and full cold levels.

Older vehicles have a large upper tank in the radiator that allow for the expansion of the coolant. As such, the normal coolant level in the radiator is never up to the neck when the engine is cold but is at some level above the radiator tubes. If the radiator is completely filled with coolant when cold, excess coolant will be spilled through the radiator cap onto the ground as it becomes hot. Enough coolant to keep the tops of the tubes submerged with cold engine operation is sufficient for these older systems. Make a note of the cold coolant level for when you may need to top up at a later time. You can verify the hot and cold coolant levels by leaving the radiator cap off and allowing the engine to reach operating temperature while idling in your shop. Do NOT remove the radiator cap when the engine is hot!

Many vehicle owners add their own overflow tanks to older vehicles (a 24 oz / 710 ml beer can works well) if a modern expansion tank cannot be retrofitted. It is not necessary or recommended to completely fill the radiator of an older vehicle with a retrofitted expansion tank. The beer can is only there to contain any spillage.

Do not add 100% ethylene glycol to top off the cooling system as this will increase its concentration in the cooling system and reduce its cooling ability. Either use commercially available premixed coolant or mix your own. Obviously, for a 50% (vol) concentration, mix equal volumes of each.