Xsara Picasso 2.0 Hdi

[Whooji]

I'm losing coolant no obvious leaks. Need to keep topping up tank. No signs of any water in the oil. Lots of bubbles in the expansion tank. Any idea's? 

[paul.h]

Bubbles in the expansion tank could be

- the expansion tank cap is not holding pressure allowing the coolant to boil

- the engine is overheating, possibly due to the low coolant level (is the temp gauge normal, does the engine fan come on ?)

- the cylinder head gasket needs replacing - is the exhaust showing more steam than normal, bearing in mind a diesel would not normally have any. A cylinder pressure test ought to show if the gasket has failed.

- look at the water pump to see if this is leaking behind its pulley