ShotgunWillie

Strangely the issue stopped happening. I believe there was air in the system after the water pump change. Whenever the coolant system got up to temperature and pressurised the level in expansion tank would drop as that air got compressed. It would surge up again when cap was removed but never quite to the same level. I think eventually the system managed to "self-bleed" the air but it took 10,000s of km to do so.

Yes its constantly on except for a short while after i top up. After posting earlier today, I topped up the coolant whilst car was cold to half way between the 2 lines, took about a litre, and stuck a piece of paper under the expansion tank. The coolant warning came on after about 10 miles of driving. 30 miles later I got a chance to look at it : no coolant on piece of paper - so hasn't come out of expansion tank overflow. no coolant in the expansion tank. I opened the lid and when pressure released the coolant level came back up to similar level as I had filled it to. I got one small drip from the overflow whilst doing this.

My 2016 Superb III 2.0 TDI 150ps, has started losing coolant since aroud 150K km when I had timing belt / water pump replaced. I dont know if its related or coincidental, the old water pump was seeping a little before this. Its now up to 200K km. The system appears to hold pressure in that any time I open the cap its still pressurised even when coolant is cold. I can't see any sign of coolant leaks, in fact I'm still not quite sure where the coolant is going, although I suspect its going out the overflow in the expansion tank. It appears to lose coolant when I top it up and then seems to find a level below the minimum mark where it is fairly constant. If i don't top it up it doesn't seem to lose much more, if i do it seems to lose most of the top up after a few hundred miles. I left it for quite a while and it took about a litre of coolant to get it back up to the max mark cold. On no occasion have I ever seen the temp gauge move above 90 deg mark. The heating does work but I can't be 100% sure its as strong as it should be. The coolant looks the correct colour and theres no gunk in oil. The expansion tank is marked G13 "mit silikat ", but I can't access the bag as its behind plastic wall, so cant confirm if its intact. I haven't had system pressure tested or sniff tested. I have replaced the cap but with no effect. What would you do next ?