My 2004 E55 consumes alot of coolant, about 0.5L per 200 km. Ive let my mechanic try to figure out what the issue is and he found out there is no external leaks anywhere, pulled the sparkplugs to check inside the cylinders but looks fine same with sparkplugs, checked motoroil and trans oil but didnt find any coolant mixed in any and also did a gass sample test from the coolant reservoir to see if maybe headgasket is leaking from combustion chamber but the levels were also normal.

Car holds pressure when cold but loses it slightly when warming up to operating temp

At this point i have no idea what the issue might be im hoping that somebody here knows what is up and would help me with this as its getting quite frustrating to have to fill up coolant very often...
The car was like this when i bought it and i started to mod it with a new IC pump, PLM heat exchanger and split cooling to see if it would fix it but no luck whatsoever...

Current mods on the car are:
VRP split cooling (no need to fill up reservoir)
PLM heat exchanger
Belt wrapkit
Bosch 010 pump

 

I have also replaced the coolant reservoir cap but it also didnt seem to make a difference