I have a 97 Passat TDI. I'm losing coolant from somewhere. I cant find the leak. Just recently replaced timing belt and water pump. I have no oil in the coolant and no water in the oil and its not smoking so I dont think its the head gasket. It has never ran hot. I dont see any drips when its parked and running. It seems like I only lose it when its running down the road. Could it be a bad thermostat causing it to heat up and spew out the overflow? Help please