What was found
TAXIING FOR T/O ENGINES RUNNING, APU & BLEED AIR ON, ALL AVIONICS ON, THE CABIN & COCKPIT FILLED WITH SMOKE. SMOKE STOPPED WHEN I SHUT EVERYTHING DOWN AND TURNED ELECTRICAL POWER OFF. A LARGE AMOUNT OF WATER DRIPPING FROM BOTTOM OF APU. INSPECTED ENGINE INTAKES, APU INTAKE & EXHAUST, NO DISCREPANCIES NOTED. REMOVED WATER SEPARATOR & INSPECTED, NO DISCREPANCIES. OPERATED LT &RT ENGINES FOR 45 MINUTES WITH ALL AVIONICS OPERATING, NO DISCREPANCIES. OPERATED APU & WITHIN 30 SECONDS OF TURNING APU BLEED ON COCKPIT FILLED WITH SMOKE. SMOKE WAS WHITE WITH A DISTINCT OIL SMELL. TURNED APU BLEED OFF AND SMOKE DISSIPATED. SHUT LT & RT ENGINES DOWN. TURNED APU BLEED ON AGAIN & SMOKE RETURNED. APU CAUSE OF SMOKE