of the picture (stolen at random from the interweb)...
So, how does it work then ?
When the engine heats up the water expands and pressure builds up
inside the cooling system. Then the radiator cap lifts a bit against the
spring to allow some water to escape and prevent the pressure getting
too high (otherwise it will blow the hoses off). Thus, during normal
operation, you can expect the level in the expansion/overflow bottle
to change from cold to hot... maybe by half an inch or so.
When the engine cools down the water starts to contract, and creates
low pressure inside the cooling system. When this happens, the radiator
cap acts like a one-way valve, and lets the low pressure draw water
back into the engine from the expansion tank. You will know that this
system is working if you open the radiator cap and find that the radiator
is full right to the top with coolant.
The expansion tank is marked with high and low water marks. When
the engine is hot the water level should be under the top line. When it
is cold the level should be above the bottom line. Water will not come
out the overflow tube and dribble down the inner guard until the level
is above the top line. But if you fill the tank to the top line when the
engine is cold, you can have some come out the overflow as the engine
heats up and pushed extra water into the tank.
Steam, foam, froth or creamy liquid in the tank, or steam/hot water
puking out the overflow tube, is always a bad sign. You should take a
look to find the cause and get it fixed.
Cheers... jondee86
