seat alhambra smoking from oil filler cap what could cause this.?

My dad has a seat alhambra 1.8l turbo petrol, the oil pump stopped working so the low oil pressure light came up on the dash my dad kept driving it then told me about it once the engine sounded noisy, i replaced the oil pump and the engine runs lovely plenty of power and no noises but once it is hot smoke comes from the oil filler cap and is noticeable when sitting at traffic lights or in traffic, i presumed this was some kind of overheating problem so i removed the thermostat in case it was faulty and to improve coolant flow i have also replaced the water pump, while in severe traffic it also starts to pour white smoke out of the exhaust. I have done a compression test and all cylinders where at 10bar which is correct for this car there is no water in the coolant or oil in the coolant it is not losing water and as apart from the oil doesn't seem to be disappearing either. can anyone help?