pressure in expansion tank

[e30-320i]

i have noticed over the last few days that if i take the cap off the expansion tank when the engine is cold it hisses and you can hear the pressure being released.what could be causing this the car still runs fine and doesn't overheat???

[alane]

i think thats pretty normal i get a slight hiss never causes me any problems car runs fine and no overheating ,

[Turbo-Brown]

As a guess, when the engine gets hot, the pressure releif valve will probably let some excess pressure out. This can't then get back in when the engine cools so the hiss when you remove the cap could be air rushing in rather than out.

[e30-320i]

yeah that makes sense maybe i'm just paranoid thanks for your help guys