Hi, I seem to be having a problem with my 110hp HDi Xsara. I have to top up the coolant frequently and it seems to be leaking out of the top of the expansion tank, through the cap. As soon as the engine is started, water disappears from the tank and it builds up with pressure. When the cap is then taken off, you can hear the pressure released and the water comes back. If the cap is taken off the expansion tank and the engine revved, the water boils over from the expansion tank. I have been told it could be the head gasket, does anyone else know what it could be? Cheers.