What was found
DECENDING, AFTER A 3 HOUR FLIGHT, THE CREW SMELLED FUMES WHILE PASSING THROUGH 8,000 FEET. FUMES WERE ALSO EXHAUST TYPE FUMES. THE CREW TURNED TEMPERATURE CONTROL TO FULL COLD. THE SMELL DISAPPEARED. LEVEL AT 4,000 FEET ON DOWNWIND LEG , THE CREW SELECTED INCREASED HEAT ON TEMP CONTROLLER. THE FUMES RETURNED AFTER A VERY SHORT PERIOD OF TIME AND THE COCKPIT STARTED FILLING WITH SMOKE. THE CREW DONNED O2 MASKS, DROPED PAX O2 MASKS AND DUMPED PRESSURIZATION. AFTER TURNING TEMPERATURE CONTROL TO FULL COLD, THE COCKPIT REMAINED CLEAR OF ANY FRESH SMOKE.