Hopefully i can help..
I note you say internal coolant loss? what tests / checks have actually been done upto this point?
Any signs of cross contamination / excess smoke,water from the exhaust / engine registering overheating / how much coolant does it actually loose?
70.000 500m loss only
Ok..considering you dont mention that no tests/checks have been done upto this point..that would have to be the next step to find out exactly where the coolants going..they need to perform a coolant system pressure test aswell as dye test on the coolant system..to check for exhaust gas in the coolant system..that would certainly verfiy wether the headgasket has gone or not or egr cooler fault...the pressure test on the system has to be done to highlight whether you have an external leak or an internal one if the pressure on the test guage falls..building the pressure up in the system to can help route out any leaks on the coolant system itself if the leak is very minimul or only appears generally under hot conditions or when the engines upto temp - unless otherwise spotted visually after checks or inspections have been made on the entire coolant system pipe work/housings...So the next step from here certainly anyway has to be the tests above...and see whats found..I do hope this helps
will carry out dye test; pressure test loses pressure;no external leak ;oil clean;no excessive water from exhaust; poss egr cooler ? is that common.
Yes the egr cooler itself or normally the pipe leading to it can leak..both of which are fairly common all round.. sometimes people think its the headgasket but turns out its actually the cooler or very minute leak somewhere else - depending how the coolers gone of course which isnt easy to diagnose being sealed unit unless there is actually coolant trace in the egr itself,chamber,could be very fine hairline crack internally which can leak coolant into the exhaust system or the egr exhaust chamber and thus when started up or running a while you get prolonged vapour from the exhaust itself,coolant type smell...the later with the pipes leading to the cooler you can often noticed that when pressure is in the system or during the pressure test you will feel wetness around there or even residue from the coolant/antifreeze itself..it "can" be possible to plug both coolant lines on these..taking the egr cooler out the circuit and then see if the coolant level continues to drop...Have you doube checked the the expansion cap itself just incase it not providing a proper seal..also maybe a quick check if you can the heater matrix just to be sure coolant isnt going..I do hope this helps