I had similar problem
Car is Audi a3 Quatro 184bhp tDi
Found was loosing coolant intermittently
Changed coolant cap
Changed coolant resevoir
Specialist suspected overheating but my temp guage always stayed in the middle
His suggestion water pump so got that changed
Still same problem then he pressure tested it. Came back OK
Mechanic kept talking about overheating. I wasnt convinced I suspected overpressure.
After a journey. Coolant level would be fine the next morning it would have gone. So i decided to leave the bonnet open let it cool down properly. Still coolant loss
Mechanic now talking EGR etc expensive....
Then i tried something that was suggested on the group blocked heater matrix.
Mechanic ggoedo you have a leak in the footwell. I didnt
So I drove around for a couple of weeks NOT turning my heater on. Bingo no coolant loss
Got heater matrix changed and seems good now
So blocked heater matrix or partial blockage was causing a build up of pressure that eventually had to release through the cap via the hole in the tank
Got genuine heater matrix from TPS. It a valeo. Autodoc sell it much much cheaper. Mechanic said dobt but it from Euro car parts as it doesnt fit
