Morning All! I need some advise please, a bit stumped at the moment. I drove from Reading to the Nurburgring last Thursday, about 380 miles. After about 320 miles into the journey, in Belgium, the low coolant level alarm/alert went off. I pulled over when it was safe to do so after a few miles. The alarm went off when going downhill on a piece of motorway which was restricted to one lane with no safe place to pull over. I noticed in these nerve-wracking few miles that the temperature gauge moved up and down but mainly stayed down around 60-65C. On uphill sections the alarm reset and the flashing thermometer stopped flashing, leading me to believe there must at least have been some liquid in the expansion tank.
Anyway, when I stopped there was still some coolant in the tank but it was very low. I topped it up with about 0.75 litres of water and set off again. On arrival at the 'Ring (about 60 miles on) I let the car cool right down before adding maybe another 0.25 litres, to slightly over the max line on the tank. Skip forward to Sunday, the first time I used the car since Thursday - left Germany for home. On arrival at the Channel Tunnel, after a 270 odd mile drive, the coolant alarm went off again. I let it cool for an hour or so and topped up again with about 0.75 litres. Drove home, just before I arrived, after about 105 miles, it went off again. Let it cool overnight, added about 0.75 litres to get it to max. So on this last run it consumed the water much faster, although I was driving faster. Since then I have tried to find where the liquid is going. There is no sign of a leak anywhere, that I can see. I don't think the head gasket has gone, there's no mayonnaise-like gunge on the oil filler cap, no bubbles in the expansion tank, no apparent overheating. When I release the pressure cap after a drive there is the usual pressure equalisation, but after a minute or so of running there is no more pressure build up. The engine is still running beautifully so I'm leaning towards an external leak but just cannot see where it is. Any pointers gratefully received!!
Edited to state that the car has done 163600 miles, without missing a beat. Cambelt/water pump change done at 123000 miles.