What was found
(CAN) PILOT REPORTED ROUGH RUNNING ENGINE. RETURNED FOR LANDING. ACFT RUN UP AND INSPECTED ON NOV 19, 2012. ENGINE RAN ROUGH ON RUN UP BY MAINTENANCE PERSONAL. UPON INSPECTION -NR 3 PUSHROD TUBE WAS BENT. NR 3 EXHAUST VALVE STUCK OPEN. CYLINDER REMOVED FOR INSPECTION. EXHAUST VALVE REMAINED STUCK OPEN, NO OTHER VISUAL DAMAGE TO CAM, LIFTER AND ASSOCIATED PARTS. CYLINDER WAS RECENTLY O/H AS PART OF OUR ENGINE ON CONDITION AND PROGRAM AMO 256-91 AT APPROXIMATELY 100 HRS PRIOR TO INCIDENT. NR 3 CYLINDER HAD NO EVIDENCE OF OVERHEATING OR SCORING ON THE CYLINDER WALLS.