What kind of soot? something like this...
http://www.pelicanparts.com/bmw/tech...ensor/pic4.jpg It's normal for it to be black like in the pic, but if it's more of a sludge based soot you're talking about then oil could be getting in your cylinders, possibly through a leaky o-ring.
But i'm no expert, it's just an educated guess. I noticed you had no replies so I just thought I'd drop my 2 cents, the electrical thread needs more support!
Hope It helped