I have a 2003 Vectra SXI 2.2dti which has recently started leaking steam from the top of the expansion tank when it heats up (the bit inside that releases the pressure if its opened hot). As a result of this I am constantly having to top up the coolant every few days. I have put a new tank and cap on it but the problem still remains, albeit not as bad. The coolant still circulates as normal and there is no issue with over heating.

I was just curious as to what might be causing this and would appreciate anyones opinion.

Many thanks.