I am losing coolant with no apparent leaks, about 1 liter per week. Pressure test does not show leakage, cap has been changed. Cold start is difficult like it is running on 2-3 cylinders. It will idle normally after playing with the gas for a few minutes. A lot of white smoke will be seen but will dissapear after a few minutes of driving. There is no apparent oil loss and the oil is clean not cloudy, likewise with the transmission. What could this be?
Sounds typical of a leaking head gasket. You might have luck adding a small bottle of dye to the coolant, then after a day or two, search with a black light, particularly at the tail pipe. The dye will show up as a bright yellow stain. Many auto parts stores will sell you the dye and rent or borrow the black light.