Should the be ANY pressure in the expansion tank when cold?

[platofan]

My old 306 and the misses's corsa does not hiss one single bit the morning after the car was last driven but the Ibiza does.

Garages hav told me this is normal but i'm getting a slight 'hiss'. Nothing too major but who else gets this on their cars?

Just looking for a definitive answer.

Thanks

 

[basssound]

Yeah there's always a little pressure when cold, it's the biuld up of pressure when the system is red hot,
without this pressure the cooling system would boil BUT it's this pressure that stops it boiling.
Under a pressure, water doesn't boil at 100c

Nothing to worry about.

 

[platofan]

Ok thanks.

Just wondered why the other cars don't have any pressure in them when cold. Maybe it's just Seat's.

 

[mgrays]

Yeah there's always a little pressure when cold, it's the biuld up of pressure when the system is red hot,
without this pressure the cooling system would boil BUT it's this pressure that stops it boiling.
Under a pressure, water doesn't boil at 100c

Nothing to worry about.

Not really true.

Closed system with no cap (this is just for example and no one makes a radiator system like this) .. if you ran the engine the pressure would get up and then when cold it would return to no pressure again - nothing goes in or out = no pressure change so no noise when you open the cap

Closed system with cap with the expansion pressure tank system fitted so a small amount of air above the level .. it runs .. when it get hot a small amount of air is expelled due to vapour pressure balance of air/steam. When cold everything goes cold again and the air that is expelled is missed creating a partial vacuum (and cap setting will not let it back in) so then on opening a cold engine there is a rush in of air to replace that expelled before. Perfectly normal.

With a car without air in the expansion tank and water brimmed right up.. then it will give a smaller gasp of air being drawn in I suspect .. so hence you never heard it on other cars and they probably had a non pressured expansion tank with a seperate radiator cap.

Good = little gasp as air is drawn in
Not got hot = no air movement
Bad = little rush as air is pushed out

The bad part happens when the cylinder head gasket is leaking so that the high pressure combustion gases (must be over 1000 psi for diesel and a bit less for a petrol) overcome the radiator cap pressure (usually 1 bar or 14.5 psi). When it cools down there is still pressure left over and so it rushes out.

Oil pressure leaks into water happen as it runs at 100 psi on the pressure side.. but then it is 0 psi on the crank/drain side. So milky/gunky water = leak from oil pressure system, Milky oil = leak into crank/drain from water .. or too many short trips in cold weather leading to condensation build up.