What was found
(CAN)PRIOR TO DEPARTURE, APU AUTO SHUT DOWN, EMITTING CLOUDS OF SMOKE. APU WAS MEL'D AND AIRCRAFT DEPARTED. ON CLIMB, CABIN AIR CONDITIONING SYSTEM DISCHARGED OIL VAPOURS. AIRCRAFT RETURNED TO BLOCKS AND C OMPLETED AN OVERWEIGHT LANDING. OVERWEIGHT LANDING CHECK CARRIED OUT AND APU REPLACED. COLOESCER BAGS REPLACED AND AIR CONDITIONING PURGED.