I own a 2017 Jaguar XJ R-Sports V6 SC AWD.
It started a month ago, the car is always low on coolant, if I fill it in the morning, the coolant reservoir tank will be empty by the evening. I don't smell any coolant burning or see any leaking coolant, nothing around the water pump (ik that's the issue most of the time). I did a full flush and added new coolant. Now I am still losing coolant, but not that quickly. What else can it be?

 

You need to replace the coolant pipes. In the search box type in coolant pipes and you will find several threads dealing with issue. They are a known failure issue.

 

Watch the Video series same engine

 

Do a pressure test when cold. But as posted above the plastic cooling parts are a prime suspect. You will be better off by planning a big repair/parts swap to the metal versions. Otherwise you run the risk of going back in multiple times to keep fixing coolant problems.

Now the good thing is we have a massive amount of threads with pictures on this.

You sound like you do have some DIY skills? Can you do any of the work?
No miles where posted and any details are most useful!
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The Parts don't cost much, pull the supercharger replace all the
metal parts you see in the videos, go to my channel and check the other
Videos, it will help you ,best thing after you finish
No more leaks