John gives a good explanation...
Be sure and replace all with new plugs.
I would also check the injectors to verify that there is not any over fueling which could also give a sooty-er than "normal" glow plug.
Uncle "Richard" Douglas has a Land Rover with big wheels that never gets stuck... until he breaks something so it won't go. Uncle Douglas always breaks something. - Anna Crowther at the Conclave 2012 (AKA Carburetor Neck)
"What's with this death wobble, Uncle Douglas, I can't keep it in 1 lane?"
UD: "Just Power through it man!"