What was found
DURING SCHEDULED ANNUAL INSPECTION, OIL COOLER REMOVED TO PERFORM A WELD REPAIR TO A CRACKED MOUNTING FLANGE. AFTER REINSTALLATION AND COMPLETION OF THE ANNUAL, A POWER RUN UP WAS SCHEDULED. AFTER TAXI OUT AND UPON POWER APPLICATION FOR THE RUN UP, THE COCKPIT FILLED WITH SMOKE. ENGINE AND ALL COMPONENTS WERE SOAKED WITH OIL. REMOVED AND DISASSEMBLED TURBINE OIL PRESSURE LINE CHECK VALVE. A SMALL PIECE OF GASKET MATERIAL WAS FOUND LODGED IN THE CHECK VALVE. IT IS ASSUMED THAT THE CHECK VALVE REMAINED OPEN AFTER THE LEAK CHECK RUN UP WHICH OCCURRED SHORTLY BEFORE THE SCHEDULED FLIGHT. THE CHECK VALVE WAS CLEANED AND REASSEMBLED. ALL DUCTS AND HOSES CLEANED. RUN UP AND FLIGHT SUCCESSFUL.