My trusty Passat seems to have started to play up, I have noticed lumpy idling for quite a while now, but recently this has developed into a misfire all the time at low revs and great lumps of black smoke when accelerating. If travelling at speed on the motorway it seems to settle down quite a bit, but there is still a noticeable puff of smoke in the reflection of lights behind me - any ideas anyone, any help would be greatly appreciated.