What was found
(CAN) DURING CLIMB AFTER A RETARDANT DROP ON A FIRE, NR 3 ENGINE BMEP BECAME ERRATIC AND A THUMPING WAS FELT AND BACKFIRE HEARD. THE REMAINING RETARDANT LOAD WAS JETTISONED AND THE NR 3 ENGINE WEAS SHUT DOWN AND THE PROPELLER WAS FEATHERED. THE AIRCRAFT RETURNED TO BASE. DURING INVESTIGATION BY THE MAINTENANCE CREW, IT WAS DISCOVERED THAT NR 14 CYLINDER EXHAUST EAR WAS CRACKED. THE CYLINDER WAS REPLACED, ENGINE WAS GROUND RUN AND THE AIRCRAFT WAS RETURNED TO SERVICE.