AUDI A4 1.9 TDI This one comes down to No pints for a month vs No pints for a year. I originally had a coolant leak through a tear in what looks like an emergency pressure release. It is a mushroom-shaped hollow rubber knob on the passenger side of the engine. The mechanic replaced that along with an oil/filter change. The following day, the coolant level had dropped again. I thought, ok just topping up from trapped air and thought nothing of it. This morning, the engine wouldnt turn. Eliminating the battery, I checked the oil and found a grey oil/water emulsion. The easy conclusion is that the head gasket is gone. However, the engine had never gone above 60 deg C while the coolant was leaking, the oil was black before it went into the garage and there is no blue in the exhaust so I am wondering if it is possible to feck up the oil cooler whilst changing the oil filter ?