Hi Paul

Regarding Glow Plugs

This is not BMW specific but if there is a glow plug error then more particulates will be produced during the warm up phase and this will contribute to the DPF getting blocked prematurley and more often.

Some systems may not be able to regenerate if there is any DTC's stored in the ECU.

The only other thing I can think of at the moment regarding glow plugs and regeneration is that for example, when you carry out a forced regen on VW cars you are instructed to activate high load consumers ie, heated front and rear screens, headlights and so on.
I believe this may put load on the engine and raise exhaust gas temperatures to aid regen. So my thought is the ECU may activate the glow plug circuit as this is a good load consumer.

Regarding the smoke you are experiencing I have had many cars emitt grey/ white smoke due to a worn turbo leaking oil on the exhaust side. Sometimes oil is obvious in the down pipe and sometimes just slight oil traces can be seen bedhind the turbo exhaust impeller in the turbo housing.

My theory behind why the smoke is grey/white and not blue is to do with the oil slowly burning off at a lower temperature in the exhaust as opposed to being combust in the combustion chamber at a much higher temperature.

In your case maybe after a run and the exhaust/turbo temperature is raised all the oil could be burning off faster than the turbo is leaking and this is why the smoking stops for a while?

Last thought the EGR is not stuck open cooling the combustion is it?

(these are just my thoughts and theories in no way am I stating this as fact)

I hope this may be of some use.

James