WHILE DRIVING 35-40 MPH, THE TRUCK STARTED SMOKING. DRIVER PULLED TRUCK TO THE SIDE OF THE ROAD AND NOTICED A FIRE UNDER THE CENTER OF TRUCK. DRIVER WAS ABLE TO PUT OUT THE FIRE. DRIVER HAD TRUCK TOWED TO HOME. IT WAS DISCOVERED THAT THE PLUG TO THE COIL PACK CAME LOOSE, CAUSED UNBURNED FUEL TO COMBUST IN EXHAUST AND RUBBER AND PLASTIC COMPONENTS TO BURN, INCLUDING PLASTIC FUEL TANK. *AK *ML