A B727-200A ON CLBOUT AT 4000 FT DECLARED AN EMER SHUTDOWN #3 ENG AND DIVERTED DUE TO #3 ENG START VALVE 'OPEN' LIGHT ILLUMINATED.

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DEPARTED PHL ON RWY 9L. ON CLBOUT AT APPROX 3100 FT MSL; THE START VALVE OPEN LIGHT FOR ENG #3 ILLUMINATED. LIGHT REMAINED ON FOR APPROX 2 MINS WHILE OPS MANUAL PROCS XYZ WAS REFED. START VALVE OPEN LIGHT FOR ENG #3 FLICKERED AND THEN WENT OUT. DISPATCH AND MAINT CTL WERE ADVISED AT APPROX XA03Z. AFTER CONVERSATION WITH MAINT CTL AND DISPATCH. START VALVE OPEN LIGHT REILLUMINATED AND REMAINED ILLUMINATED. ADVISED MAINT CTL AND DISPATCH THAT OPS MANUAL PROC XYZ WOULD BE COMPLETED. CREW DECLARED AN EMER WITH PHL DEP AND STATED INTENTIONS TO DUMP APPROX 5000 LBS OF FUEL AND RETURN TO LAND AT PHL. CREW STATED EXACT NATURE OF EMER WITH PHL DEP AND SPECIFIC REQUESTS FOR TYPE OF ASSISTANCE REQUIRED WHILE ON FREQ AND DURING LNDG. CREW RESTATED THESE INTENTIONS TO PHL TWR ON CONTACT. LEAD FLT ATTENDANT WAS CALLED FORWARD FOR ACFT EMER BRIEFING AND WAS RECONSULTED TWICE AFTER INITIAL BRIEFING AND PRIOR TO LNDG. DISPATCH WAS RENOTIFIED OF OUR SPECIFIC SIT; INTENTIONS AND WE CONFIRMED DECLARATION OF ACFT EMER. REQUEST MADE THROUGH DISPATCH TO COORDINATE EMER EQUIP ON GND IN PHL. ALSO COMPLIED WITH OPS MANUAL PROCS XYZ0. ENG PRECAUTIONARY SHUTDOWN AND OPS MANUAL ABC4 FUEL DUMPING. FLT LANDED UNDER MAX GROSS LNDG WT; APPROX 148000 LBS. PAX AND PHL STATION WERE BRIEFED. MAINT REPLACED ENG #3 START VALVE AND PRESSURE SENSING SWITCH FOR START VALVE OPEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PROB WAS CAUSED BY CHAFFED WIRING ON THE ENG AND WAS FOUND WHEN MAINT REPLACED THE START VALVE AND START VALVE PRESSURE SWITCH. THE RPTR SAID THE START VALVE 'OPEN' LIGHT WAS INTERMITTENT IN OP AND AFTER THE WIRING WAS REPAIRED NO FURTHER RPTS WERE MADE.