As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER ENGINE START SMOKE BEGAN TO ENTER CABIN. PLANE WAS AT THE GATE AND PASSENGERS WERE DEPLANED THROUGH THE GATEWAY. MAINTENANCE ISOLATED SOURCE OF SMOKE TO OIL IN PNEUMATIC SYSTEM FROM THE APU. PLACED APU ON MEL AND REPLACED WATER SEPARATORS.