I have just noticed that a hissing sound is coming from the radiator cap ?? This was noticed over the weekend (very hot) and after driving the car not too fast ;) for a few miles. Anyway having allowed the car to cool for a minute or 2 when I got home I opened the bonnet to fill up the washer fluid bottle and it was then that I noticed a hissing sound coming from the header tank. On closer inspection it appears to be the cap !! Surely this is not supposed to happen - does this mean a replacement cap is required or could it be something more serious ?

Any views would be welcome ?

Hi,

Check your rubber seal.
It does get hot and has a fair bit of pressure.