I bought a Nissan X-Trail 2.2 diesel yesterday. I noticed the in-car heater was fine at first and then wasn't getting warm. I Checked engine coolant which was low so topped up with approximately two litres of water.

When I got home nine miles later, the heater had gone back from hot to cold. I checked radiator coolant and found I had lost the same amount of water, which was being blown out of the back of the coolant expansion tank?

The car appears to drive as it should with no white smoke coming from the exhaust, the temperature guage reads normal and there is no water in the oil or on the dipstick?