Introduction:
A cooling system is always equipped with an expansion tank. This tank stores a certain amount of coolant. It also allows the coolant to expand. Due to temperature fluctuations and pressure increase, the coolant level will rise and fall. This can also be noticed when the cap of the reservoir is turned with a warm engine; a hissing sound is audible. At that moment the pressure in the system is relieved and the coolant level rises (it can even flow out of the reservoir). Under pressure, the coolant temperature can easily reach 1206F without boiling, but once the pressure is lowered (e.g., by loosening the cap), the coolant will start to boil and expand.
The image below shows how a car’s cooling system looks. More information can be found on the cooling system page.
Different types of expansion tanks:
Car manufacturers may use different types of expansion tanks. Sometimes a transparent tank is chosen and other times an opaque tank with a plastic float element. The image shows a transparent tank with minimum and maximum markings. The coolant level must stay between these markings, preferably at maximum. The advantage of this type of tank is that the cap does not need to be removed to check the fluid level. This type of tank is often used in brands like V.A.G.
Another option is an opaque tank with a float element (see image).
- The tank is usually made of black, opaque plastic. The cap must be removed to check the level.
- At the maximum fluid level, the float element (which floats on the coolant) will protrude from the top of the tank.
- At a minimum fluid level, the float element will lie deep in the tank. This is a sign that it needs to be refilled.
The disadvantage of this tank is that the cap must be removed to check the fluid level. This is not possible with a hot engine. If the engine is too hot, the fluid will flow out of the tank because the boiling point is lowered and the coolant will expand and may start to boil. If this is not the case, but a hissing sound is audible, then the level will increase due to the pressure drop. At that moment, it seems as if the fluid level is at maximum, but if, for example, an hour is waited for the engine to cool down, the fluid level may only be halfway up. This can give a distorted view of the fluid level. It is therefore always important to check the level with a cold engine.