The filler tube is located front and center in the engine bay, just behind the radiator.
Originally this was a 20psi pressure cap.
The cap on the remote header tank was originally 15psi.
Both caps, when ordered through Jaguar, now come as 16PSI caps, regardless of the initial claim in the shop manual that both caps are not interchangeable.
On the XK engine I donāt know, on the early V12 you are correct, but on the later V12 with the addition of the atmospheric catchment tank the header tank is always full and excess coolant goes to the atmospheric catchment tank and then gets sucked back up.
Indeed Patrick, and thatās where you should fill the system from ad the header tank cap is slightly lower than the filler cap.
Yes, an unnecessary complicationā¦
I would be curious to know if your vehicle has the removable access plate behind the front left wheel wellā¦where the atmospheric catchment tank resides on US spec cars ? If it does, I might think about adding the catchment tank and associated piping. Could you possibly have a look see ??SD Faircloth
Patrick, the black plastic item is the catch tank. Itās just not translucent.
Horizontal part steel tube goes in, vertical hose in p clip vents the final excess to the ground.
I now see I must have had a leak somewhere, in a fully sealed system there wouldnāt be any air in the expansion tank. Makes sense. It will still be overfilled initially, and come out the catch tankā¦ right?
On my uk spec. the tank was bolted in exactly like that. No cover, or anything.
I checked and itās all in the workshop manual. The charcoal is on the right and the atmospheric tank on the left side. At least according to the book. So nothing should be in the way.
The inlet tube should reach to the bottom of course and Iād say the atmospheric tank is a really good addition to the cooling system.
**
The pressure in the cooling system is the same everywhere, Patrick; two identical caps is meaningless - unless both places has an overflow tube connected to the recovery tankā¦
The filler tube should have a blank, or possibly a higher rated cap - acting as a back-up if the header tank drain is cloggedā¦
Frank
xj6 85 Sov Europe (UK/NZ)
Doesnāt matter what its rating is, there is no relief tube so no way for it to relieve pressure.
Twin rubber seals important then? If the spring is just a hair weaker it would otherwise push out then. The brass disc wouldnāt be enough to seal.
I just received the original replacement part.
Look a bit different, membrane wiseā¦
The cheap knock off has a much thinner one in the flange of the cap.
Yes. Can you get any radiator caps these days that donāt have the rubber seal under the top flange?
Oh yes you can. I looked for a long time and then bought the wrong one š„²
5ā¬ Sounds like a good deal, well it would if it had workedā¦
Ah well, the original costs ā¬15,- so I can overcome my hesitations 
**
Well put, Kirbert - so why would Jaguar, or anybody else, fit anything but a cheap blank cap thereā¦?
Frank
xj6 85 Sov Europe (UK/NZ)
Better question: Why would they attach the caps to the car with little chains so you canāt mix them up? I have a feeling there was probably a heated discussion in a meeting where some insisted on different pressure ratings for the two caps and chains to keep them sorted, while the guy who designed the thing was pulling his hair out over the stupidity of it all. In the end, they probably agreed on the two different caps and the chains just to settle the disagreement.
:)ā¦
Frank
xj6 85 Sov Europe (UK/NZ)
