After renewing all the hoses on my 5.3 it seems every time I run her I get a different leak. During the course of topping up the system I have noticed the replacement rad caps are not a good fit. By not a good fit I mean they are not loose but the opposite, really hard to turn on . Extremely tight. When removed the rubber seal is stuck to the top of the filler pipe instead of staying with the cap. Maybe I’m over thinking things but is it possible these caps are on that tight the system is over pressuring and that’s why it’s blowing the pipes? Just a thought?

 

Anything is possible! they may not even be sealing well, in spite of the hard effort to get them on. These are what I use, as I could not get good quality ones anywhere else:

After that, the pressuriser tool is the only reliable way to find leaks.

 

Wow! They look a bit of an animal greg. I was very impressed with the Stant thermostats you put me onto. Quality was superb. I was planning on ordering two new caps direct from jaguar. Do you not think they would be good enough?

 

Probably will be OK

 

Just noticed this in the handbook. Wonder if it has some bearing???

 

Brinny
This is a rare piece of foolishness from Jaguar! The Great Palm deals with it in his book. The entire cooling system is connected, so when the cap on the central engine filler spout blows open, the "vented" coolant does not leave the system; it merely goes above the blow-off valve seal into the chamber in the filler spout between the cap blow-off valve and the cap top seal, in which is the outlet spigot that sends it down the flexible to the rad-top banjo fitting, and back into the main system! The cap that seals the entire system and controls the pressure is the one on the wing header tank, as this is the only one that vents the blow-off to the atmosphere - ie into the wing atmospheric tank. In fact, the pressure should in theory be identical BOTH sides of the filler spout cap blow-off valve, as both sides are in the same system, even though in practice there might a localised pressure difference.
Before I redid my coolant system with a header tank higher than the engine and radiator, thus eliminating the banjo/bleed pipe, the filler spout and about 14 leak-prone connections, I ran a solid cap on my filler spout top fixing, It does not need a blow-off cap at all. for instance:
https://www.carbuilder.com/uk/radiator-cap-valveless
Attached a good read on the system and how I did it! This mod is more difficult, maybe even impossible with later ABS equipped cars, as the space I used by relocating the washer bottle is taken up by the ABS stuff.

 

This is interesting - I went to go for a drive the other day, started up the car and went inside to get my phone... I never drive an XJS without a mobile LOL and noticed a large puddle of coolant under the car.
I soon worked out it was coming from the radiator cap.

I waited until it cooled down, took the cap off, wiped both surfaces and put it back on.
Started the car, no leak, went for a drive... no leak. Waited for it to cool down again and removed both caps - they were both full...

Has this coolant come from the expansion tank in the wheel arch?

Radiator cap is 2yrs old.

 

It depends where the "leak" was. If behind the LHS front wheel arch, then the atmospheric overflow and no worries. If below the header tank or elsewhere, a leaking hose, a leaking cap, or possibly a blocked line to the atmospheric overflow tank.