So here is the very puzzling scenario.
When driving along there are absolutely no problems, no smoke whatsoever. In fact, in park or neutral, the car still drips water out the exhaust which indicates clean combustion. I've let the car idle for 10minutes and no sign of any problems. Temperatures are fine and even with revving a bit and spooling the turbo, no smoke at all.

BUT, when in drive or reverse and idling, for some reason the revs dip slightly and the car has a bit of a loping idle, then after a few seconds white smoke (not steam) starts puffing out the back. The smell is not very strong of anything (so doubt its coolant).

Here is what I have checked or done to troubleshoot:

Car was serviced 5 months ago with new oil and filter, so just changed the oil again. Used fully synthetic 5W30, which did not change the situation at all.

Dismantled the pipework leading from the turbo to the intake, there are small signs of oil especially in the intercooler pipes, but nothing unusual for a turbo car.

Tried to follow all the vacuum pipes, but the maze got a bit confusing after a while and could not exactly deduce anything, but all pipes seemed to be in a good condition.

The only "modifications" that are on the car are a pipercross filter and middle section of the exhaust that was replaced by a stainless steel piece without a silencer/muffler. But both of these have been on the car for almost a year with no problems.

There is no loss of coolant, or oil and no signs of mixing anywhere.

This issue "seems" to have started a few months after the last service, but I am baffled by it.

This only happens when the car is in drive or reverse, but the moment you tap the accelerator or just raise the revs slightly, there is no problem at all. If it puffs for over 15 or so seconds, then when you accelerate a huge cloud of smoke billows out, can almost block out the car behind! So it seems that whatever is happening, builds up some incorrect mixture and can get more and more the longer it stays in dipped revs.

I have tried to locate any sensors in the intake pathway that could be dirty, and sprayed carb cleaner down any "unopenable" channel, but did not solve it either. Are there hidden sensors in the throttle assembly?

Also the other thing I was wondering, what controls the idling level that could be different when it is drive compared to when in neutral or park? Is it the readings from an O2 or lambda sensor?

The way around this for me is that I just shift the gear into neutral when coming to a stop everytime. So no problems at all with that, except I would like to stop the actual issue.

Any help would be appreciated....