Not enough, you have to trace further where the hiss is coming from or further develop your english skills reporting the details. Nothing personal there but it is what it is. For instance, a simple comma placed between the words 'tank' and 'cap' changes everything somewhat. Becomes two questions rather than a run-on incomplete sentence, and where you are really at anyway. One's english or more specifically the non-use or incorrect use of it can kill you here.
If cap, then old ones sometimes leak at hand tight when the gasket hardens and cuts a groove in it. You simply take like a big set of channellocks and then overtighten cap just a bit more. Can run for years more like that if you are not hamfisted.
If crack and above liquid level in tank it's new tank time.
Yeah sorry I was meaning I could hear hissing and it was coming from this coolant tank any yes I tightened the cap up and still hissing have over tightened it drove my car and the hissing is there it's definately coming from coolant tank could this be a problem with pressure ie head gasket ?
Do you see any coolant running down the inner fender? You have to carefully make sure the caps on fairley tite. Mine was doing the same thing a few weeks ago & I thought the cap was on tite, removed it & cleaned up the slight coolant around it & carefully tighted it.