Continental 0-200 engine, temperature and dewpoint close together (it was misty) rough running engine, I won't state the obvious!
Don't you all just hate it when the long distance cause of accident speculators start posting?
And re: Pompey Pauls post - yes, I have been caught out by the weather too. I consider myself a careful pilot. I don't know what the temperature and dewpoint was forecast at the pilot's destination airport if one was not available for his destination airport or even if the forecast at an airport close to the destination was different or even if the pilot obtained a weather forecast. It is easy to be wise sitting in front of a computer but a different matter when something startsto go wrong in the air and fog starts building - yes, divert! - but, the pilot has to make a decision to either land or divert to another airport where the conditions could be developing into something even worse. Fog can develop very very quickly!