GTD Coolant Drops, Cap Hissing


New member
United Kingdom
MK6 Golf 1.6TDI
Hello, I’ve just got a MK7 GTD recently and originally it had an overheating issue which was fixed by replacing the timing belt and water pump. It then would under heat, which was fixed by replacing the thermostat.

It still seems coolant issues are following me around. Locally I had driven it 400/500 miles perfectly, but today I did 2x 3 hour motorway journeys, after each journey the coolant level had dropped to minimum, the coolant cap was also very loose with little to no pressure while opening it. I also noticed a slight hissing sound.

Would I be right in thinking this is just the coolant tank cap that would need replacing? I’m assuming it’s not allowing the pressure to build, but then allowing the coolant to steam out which is causing it to drop?

Heaters work great regardless, the only 2 other common options would be the EGR leaking coolant, or intake manifold I’ve heard. Although the hissing and loose cap makes me think it could be just the cap that needs replacing but I’m unsure if something like that would cause it to lose the coolant too.