What was found
(CAN) JUST BEFORE PUSH BACK, CABIN RAPIDLY FILLED WITH SMOKE (NON ELECTRICAL IN NATURE). A/C WAS DEPLANED VIA THE LOADING BRIDGE AND SHUT DOWN TO BATTERY. MAINT FOUND THE APU OIL TANK EMPTY AND INLET PLENUM CONTAMINATED WITH OIL. FOUND APU COOLING FAN HAD DEPARTED ITS SHAFT BUT WAS CONTAINED IN HOUSING. COOLING FAN SUPPLIES COOLING AIR TO APU OIL COOLER AND APU GENERATOR ASSY. APU ASSY WAS REPLACED AND A/C EFFECTED BLEED DUCTING PURGED/CLEANED. COOLING FAN SHAFT THREADS WERE NOT DAMAGED. RETAINING NUT BACKED OFF AND THE FAN MIGRATED OFF THE SHAFT. IT IS UNCLEAR AT THIS POINT HOW THE APU OIL WAS ABLE TO CONTAMINATE THE BLEED SYSTEM. FURTHER INFORMATION WILL FOLLOW AFTER THE APU TEARDOWN REPORT IS RECEIVED