2003 vw pass at 1.8 turbo

Caution - never work on the engine cooling system with a hot engine. Suggestion: This could be a sign of leaking head gasket, if you take oil filler cap off it may show a darkish creamy color (emulsification) or your heater core is leaking into the cabin. You will notice wet carpet/feet and possibly steam coming out from under the dash and it will smell of antifreeze (unpleasant). Try blue devil headgasket repair first (don't just add it to the expansion tank, instead when the coolant is low again, with the engine "COLD" remove the upper radiator hose - engine end of the hose and poor it in there, you may have to drain a little more coolant from the bottom of the radiator. Replace and secure the hose and top up the coolant, follow instructions on bottle) and see if it helps. If it does change the engine oil so you don't have water/antifreeze (coolant) mixed in with the oil. Heater core replacement requires removing the dashboard so it's a big pain. You can disconnect the coolant pipes to the heater core and join them together with a brass coupling ($2 Home Depot). This will tell you if that the issue and you can run the car this way but you won't have heat. Good luck and again never work on the engine cooling system with a hot engine, you could be badly burned even blinded by the super heated coolant. Also don't let you dog drink/lick the coolant as it is highly toxic to all living things.