What was found
APU SMOKE: APU SMOKE IN CABIN AND FLIGHT DECK, AFTER STARTING APU AND TURNING ON GEN, PROCEEDED TO TRANSFER BLEED SWICHES HEARD A LOUD THUMP NOISE ALMOST EXPLOSIVE SOUNDING, THEN SMOKE EXPELLED INTO FLIGHT DECK AND CABIN AT SAME TIME EMEDIATLY PUT BLEEDS BACK AND SHUT APU DOWN OPEND RAM AIR. SMOKE STOPPED. SMOKE WAS OF BURNING OIL SMELL FOLLOWED SMOKE FIRE FUMES QRC, LANDED. DID NOTICE CARBON AROUND CHROME RING OF APU EXHAUST. REMOVED AND REPLACED APU, OPS AND LEAKS CHECKS SATISFACTORY IAW AMM 49-10-00.