water leak on a 986 boxster

[matjk]

By boxster has started using a bit of water, I think its leaking from the filler cap on the header tank as when i open the boot there is water vapour around the filler area and all across the inside of the boot lid. The cap is a bd to get to screw on straight, it wants to cross thread but it will go on. I have done it up tight and when the car is hot it seems to hold pressure as it hisses when i undo it. Should i change the tank or maybe just the cap, or am i missing something?
Thanks guys

[matjk]

thanks, i tried googling the part 996-106-447-04-M100 but not much comes up, will call local opc to see if they have them.