I recently bought a 1993 200sx s13 - my first car

mileage 53000 miles
However about a month and a half ago plumes of white smoke started coming out of the exhaust intermittently, not sure what the problem was I decided to check what I considered the obvious,
The car is completely standard
So far my findings are as follows:
no oil at the tail-pipe
no water/oil mix at the dip-stick
no bubbles in the radiator
After 500km - it needed no oil
After 500km - it tooks around 1.2l coolant
Also it seems somewhat heavy on fuel
Turbo is boosting at 0.5 bar according to the boost guage
Heat guage never goes past half-way
No odd engine/turbo noises - not that I have heard 1 of these before
I haven't checked the ecu for error codes yet but I will be doing that soon,
The smoke plumes are almost random, they occur on some days and not others, and usually occur around 2000rpm, for example, I saw quite a lot of smoke the other day so I pulled the car over and revved up to 4500rpm and no smoke, but as soon as it can down to idle it started smoking again and after 2min at idle - no smoke. Please advise me on what it could be, I am taking it for a cooling system pressure test next week, if it passes that, not sure what to do
thanks