On my 3.0 litre S-type jaguar year 2000, I have 3 way pipe which is part of the coolant system it has some sort of presure or bleed valve fitted in the centre is a small hole now today water is spitting out of this hole and so the coolant level is going down there are no other leaks what is this part and what is causing it to eject water

 

First of all, welcome to our wonderful world.

Yours being RHD like mine, and they are different to LHD in this area from what I read in my parts catalogue.

If its the hose that runs from the plastic nipple in the top radiator hose TO the header tank, and has a "T" just near the oil fill cap, read on.

That "T" is nothing but a "t" joiner for the other hose that goes to the throttle body.

The wifes leaked here, and I replaced that "crimped" set up with a simple standard "T" and 3 hose clamps. I also replaced the top radiator hose at the same time as that notorious plastic nipple had cracked.

I see, nor saw, any valves in that hose of any sort.