hi,

in case of engine (1.9 tdi) mixing water with oil what the possible cause of that and what should i do to fix it ?

thanks

Head Gasket maybe?

i changed it 2 months ago

+1 head gasket

most likely to be head gasket.

could possibly be leak in oil cooler?

Possibilities:-

1) Head gasket

2) Oil/water oil cooler leak

3) If you only do short trips, condensation under cam cover. In this case you might well find a mayonnaise deposit inside the oil filler cap.