Expansion tank function

There are two types of expansion tanks in modern vehicles. Some are connected to the cooling circuit via one hose, others connect in the cooling system with two hoses. Their location under the lid also varies: in some cases they are installed to the radiator, in others they are located further. Although expansion tanks differ in design and location, they all do the same. They store the coolant reserve and retain the enlarged coolant in the coolant. The high temperature of the coolant causes an increased pressure in the coolant because the hot coolant is expanded by volume.

The coolant is pressurized in the tank. When the coolant temperature drops again, the liquid is restored from the tank. In other words, the expansion tank compensates for the changes in the volume of the coolant as it passes through its heating cycle (extension of volume) and cooling (volume reduction). Expansion tanks are usually made of transparent plastic to allow checking the coolant level, and usually have “minimum” and “maximum” markings.

The malfunction of the expansion tank is usually the result of cracks or leaks from the tank itself or a problem cap.

Rarely checked but important: radiator cap and expansion tank cap
The radiator cap is a very important part of the engine cooling system, but is often neglected when troubleshooting in the system. If an individual expansion tank is installed, which is the most common case today, the expansion tank cap is just as important.

The radiator cap and the expansion tank cap provide the openings for filling the cooling system and are hermetically sealed. But they do much more than that.

The caps are designed to ensure that the prescribed pressure in the cooling system is maintained at any time during operation, allowing the air to come out in case of overpressure. They should therefore be considered as a “safety valve” of the cooling system. The coolant is under pressure because the increased pressure on the coolant increases the boiling point, or in other words, makes the system more boiling and therefore perform better. Boiling should be avoided because it creates air bubbles in the chain. Air bubbles significantly reduce the circulation of the coolant and can cause the engine to overheat.

Let’s clarify exactly how the restriction works. When the coolant is heated, it expands volume. The expanding fluid causes increased pressure in the coolant. For this purpose, the cap is equipped with a pressure release valve.

When the pressure reaches a given value, the pressure release valve opens, reducing excessive pressure by leaving the air. The expansion tank absorbs the enlarged coolant. When the coolant cools, it shrinks, which would create a vacuum in airtight closed systems. For this, the cap is provided with a vacuum safety valve. The coolant is sucked back into the coolant to maintain the correct level of the coolant. This also generates a vacuum in the expansion tank. Therefore, the vacuum safety valve opens, allowing the air to enter the tank until the pressure is equalized.

Types of caps: ventilated against non -ventilated
There are only two main types of caps, but many ways to describe them. The caps are equipped with a pressure relief and a vacuum safety valve are most often described as “ventilated caps”, while “non-finished caps” are ordinary and simple caps without special function valves. Some vehicles have a ventilated radiator cap to ensure that the system pressure is maintained at a safely set or maximum permissible value at any time. In this case, the expansion tank cap is non -finished. In other vehicles, the expansion tank serves as part of the pressure coolant system, e.g. In the case where the radiator lacks an opening for filling with coolant and the expansion tank serves as a filling point. In this case, the ventilated cap will be on the expansion tank instead of the radiator.

Below the top of each cap there is an upper main seal that seals the filling opening or the top of the throat. But only a ventilated cap will have two additional seals at the bottom of the cap: the pressure valve with a throat seal and a vacuum seal base (brass or stainless plate) with a seal centered in the safety valve.