Could be any of various faults in the fuelling system, but also possible it's been remapped badly for some baseball hatted bod, when you do see really bad smokers under power more often than not there are tell tale signs of bolt on accessory shop tat and dubious stickers giving you some idea of the mentality involved, sometimes those stickers refer to excess Diesel smoke in humourous (facepalm) terms.

Once again, lack of coppers, or rather coppers who can be bothered about such things....and yes i saw a serious smoker only last week which was completely ignored by plod who passed by seemingly without noticing.

If it's greyish smoke, it's an active DPF regeneration taking place. The soot in the DPF is being 'coked' into ash at a temperature of over 500 celsius by injecting extra diesel into the exhaust gases.

It's effectively the same as when the soot in an old chimney starts to burn. If the DPF was fairly full, the regeneration can last 10 to 15 minutes.

A crudely remapped diesel puffs out clouds of black smoke.