Sprinter 906 Coolant Tank Question

[cts1975]

I have 2012 sprinter 906 with the 2.2 cdi engine. It has started to loose water. The level drops from max - min in approx 600 miles.
I've had a look at the cap on the water tank and it has a kind 'ear' that lines up with a similar 'ear' on the plastic tank. My assumption was that these 2 'ears' should line up. However when they are lined up the cap feel a little loose and still has maybe a 1/4 of a turn till tight.
Does anybody have any experience of either the tank & cap or loosing water?
Its done 55k from new without loosing water. I'm thinking that the cap has been left loose (ear to ear) after it last periodic inspection during the anti-freeze test. I cant help thinking that the cap should be tightened past the 'ear' an extra quarter of a turn.

[btcc123]

I have no experience on this but seams wrong if the cap feels loose as I would imagine that the cap should be tight.All the cars I have ever had the cap has to be tight and I woul think a van should be no different.

Perhaps ring the dealer and ask.