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from collapsing due to partial vacuum, which would be created if there was no air entering during the cool down of the
engine and cooling system.
1. The spring pressure relieve valve, which closes off the lower sealing seat in the filler neck. This valve allows
pressure to build up to a specified level, and protects the cooling system from damage due to over-pressurization.
When exceeding the designated range of the pressure valve spring, it permits excess pressure to escape through the
overflow tube (providing the cap is functional).
2. The vacuum relief valve, which in the open system, allows air to enter as the coolant cools.
THE RESERVOIR OR CLOSED SYSTEM
The following disadvantages of the open system brought on the development of the reservoir or closed system.
1. Since the coolant expands every time the temperature rises, these open cooling systems are designed to operate
with less than full radiator tanks. The excessive coolant being expelled through the overflow tube until proper operating
level is achieved. This leads to less coolant in the system left to do its job. This and the fact that there is air in the
system reduces the efficiency of the cooling system and can create overheating problems.
2. Cars today tend to have smaller engine compartments and smaller hoods. Therefore, the open systems which were
generally larger to allow for coolant expansion, needed to be replaced with the closed cooling system. Closed systems
tend to be smaller.
The closed system consists of a special type of pressure cap and a reservoir tank. The pressure cap has an additional
sealing seat directly beneath the cap. This enables the cap to sit on the upper sealing seat of the filler neck. The
reservoir is connected by the overflow hose from the filler neck, with the hose extending below the normal liquid level.
In the closed system, contrary to the open system, the system is filled completely with coolant and the reservoir is only
half full.
When the engine starts and begins to warm up, the coolant expands and is forced out through the pressure valve into
the reservoir.
When the engine stops and cools down, the vacuum created in the radiator by the contracting coolant and the upper
sealing seat of the cap allow coolant to be drawn back into the system from the reservoir.
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