OWNER WAS DRIVING WHEN SOMEONE ON THE SIDE OF THE ROAD POINTED AND INDICATED THERE WAS SOMETHING COMING OUT OF HER VEHICLE THAT LOOKED LIKE FOG. SMOKE WAS THEN NOTICED AND HER HUSBAND GOT OUT OF THE VEHICLE, LOOKED UNDERNEATH, AND SAW FIRE. HE BEGAN THROWING SNOW ON THE VEHICLE. FD EXTINGUISHED FIRE. SUBSEQUENT EXAMINATION REVEALED THE EXHAUST SYSTEM OVERHEATED AND IGNITED THE FIRE DUE TO AN ENGINE MANAGEMENT PROBLEM - SPARK PLUGS SHOWED OVERLY RICH FUEL/AIR MIXTURE.