WHILE DRIVING ON INTERSTATE HIGHWAY I OBSERVED SMOKE BILLOWING IN MY REAR VIEW MIRROR. I PULLED OFF HIGHWAY AND FOUND A FIRE IN ENGINE COMPARTMENT WHICH I EXTINGUSHED USING AN EXTINGUSHER. THE FIRE WAS CAUSED WHEN A PLUG AT THE REAR OF THE ENGINE FELL OUT CAUSING THE ENGINE OIL TO EMPTY ONTO THE EXHAUST SYSTEM WHICH CAUSED THE FIRE. THE TEMPERATURE AT THE TIME WAS WELL BELOW ZERO. I WAS TOLD THE PLUG IN THE ALUMINUM BLOCK ENGINE WAS MADE OF STEEL WHICH HEATS AND COOLS AT DIFFERENT RATES AND APPEARS TO HAVE CAUSED THE PLUG TO FALL OUT. THE CAR WAS A TOTAL LOSS PER MY INSURANCE AND VOLKSWAGEN. THE CAR IS A 2003 VW PASSAT WAGON GLX 4-MOTION. *AK