expansion tank....

[666damned]

something has been bugging me for a while chaps...but i cant fully understand the point of an expansion tank.........bear with me......i understand after a a spirited drive the temp/pressure within the cooling system could increase to such an extent that it will overcome the pressure resistance of the radiator cap,and overflow into said tank...right,this coolant can only sit there,as the 'vacuum' effect of the coolant during cooling within the engine this tank full of coolant can't get 'back in' to the system due to the pressure of the rad cap(now greater than in the system) and will stay there.............am i correct........or having a blond moment...lol..........over to technical guru's to put me right........

[Nick Mann Services]

All correct, except the vacuum should be pulling the water back in. That is the reason for the pipe that drops down in to the expansion tank from the tanks cap.

[666damned]

yeh totally agree with you there nick.but without going to car to look ,i'm sure the return pipe is above the rad cap seal which is under spring pressure.....no? and therefore the vacuum in the system would 'hold' this seal down with the 'extra' spring pressure above.........starting to confuse myself now...pmsl.....

[Nick Mann Services]

The rad cap has several seals on it. I can't say I've spent time studying it, but there must be a way that it draws coolant back in. The water level definitely rises when warm and drops when cold.

[swinks]

There is 1-way valve in radiator cap, opening under particular pressure in 1 way: from radiator to expansion tank, open all time in return. Coolant return is based on basic physics: liquids in connected vessels, hence hose attached to expansion tank cap and going down tank. And because of this, it's so important not to leave expansion tank empty.