After an easy ride ( less than 130 km/h), I parked my jag with the front wheels 50cm lower than the rear wheels. There was water vapor visibile, so I opened the hood.
I saw coolant spilling out of the cap of the fluid reservoir.
The next day, with the engine cooled down, the fluid reservoir shows about 1liter of fluid missing.
I had no warning signs or high temperature display.
What can cause this ? Can I diagnose something with SDD ?

Was the coolant coming out of the cap, or from where the small coolant pipe enters the header tank ? If it really was the pressure cap, there is something wrong with the cap, or there is a crack somewhere near it. These header tanks are notorious for cracking and leaking.

+1 above but try new cap first.

There are no cracks. I replaced the fluid reservoir two years ago.
It looks like there's much pressure, which blows the water through the cap.
I'm in fear, my head gasket is broken, which would be a surprise, as I only ran 110.000km.
Are broken head gaskets common to the V8 ? ( I never read much about this.)

Head gasket failure pressurises the coolant jacket and causes coolant to be blown out past the pressure cap. I had this with a Series 3 XJ6. One symptom when the engine has cooled down is pressure remains in the coolant jacket when there should be none at all. So with your engine cold, remove the pressure cap and if you get a "whoosh" noise of pressure being release, then a failed head gasket could be your problem.

It must be quite recent because t he coolant will be being forced out the whole time you're running the car, so you should eventually get a low coolant warning. Other thing to make sure is to get a shop to check the coolant for combustion by-products.

I believe HGF is not common on these engines, but can occur especially if the engine gets overheated. I presume you have not had any low coolant messages on your dash display.

I agree HGF is not common but we have two people on this forum dealing with it

at the moment.

If there hasn't been any overheating it's really bad luck.

https://www.jaguarforums.com/forum/x...8/#post1888437

I did some futher testdriving and now everything seems to be ok.
No temperature above 95° ( SDD logging). No water comming out of the fluid reservoir.
I think I overfilled the fluid reservoir, as I refilled with the car not standing leveled.
Why this fault leads to looses water below minimum is not understood by me.

Good- a happy ending!