Hi!
The coolant cap on the cross tube, used for filling, occasionally leaks from the top seal and sprays a little water out and then leaves white spots as it evaporates.

I know that the pressure is really set by the cap on the overflow tank as the filler cap really has no where to release excess pressure.

It seems to me that the overflow style filler cap top seal is a low pressure seal and under vacuum, to suck in water as it contracts, but is unable to fully seal if the 16lb pressure is exceeded.

I cannot seem to find a sealing cap, so should a higher pressure cap (e.g. 20lb) be used on the filler so that it does not open at all, and leave the pressure management to the cap on the header tank?

I have inspected the top seal and the brass of the filler tube and there do not appear to be any cracks and it does not seem to be uneven.

I have tried different caps and swapped both and that just changes the frequency of release. Sometimes it does not happen at all, but depending how it is replaced, seems to affect the result.
Both caps as 'recent' (least 4 years) Jaguar 16lb. Any reason not to use a higher pressure to 'seal' the filler cap location.

 

Good diagnosis, well done, have a drink, or 2.

Couple of things I have found over the too many years.

The filler pipe cap can be a mongrel to seal, and that can be the lip of the tube is not as good as it should be, so some careful rework generally sorts taht.

The actual "upper" seal in that cap is not as soft as needed, and that will allow escape.

The small hose hose running aft of the header tank, the one going TO the atmospheric tank DO BLOCK UP, and that prevents pressure blow off by the pressure cap. Also the end of that small hose can sit in the mud in the bottom of that same atmospheric tank, and also cause pressure issues.

The pressure cap is a dud. I had that, and I went thru 10 caps at work, with my tester, until I found one that was even close to spec.

 

OK, installed 20lb cap on the filter neck and no leaks now. Keeping the 16lb on the header. Am happy and will have those drinks promised. I can understand why originally they had two different caps originally. :-)