A B737-500 IN CLB AT 17000 FT DECLARED AN EMER AND DIVERTED DUE TO #2 ENG PWR STUCK AT 91% AND REQUIRING INFLT ENG SHUTDOWN.

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FLT XYZ LGA TO HOU MAY/XX/98. CLBING OUT OF LGA; WE RECEIVED AN INTERMEDIATE LEVELOFF AT 17000 FT. SINCE THE FMC HAD PREVIOUSLY FAILED; THE AUTOTHROTTLES WERE NOT ENGAGED. WE WERE MANUALLY ADJUSTING THE THROTTLES AS NEEDED FOR THE VARIOUS FLT MODES. THE L AUTOPLT WAS ENGAGED; AND WHEN IT PITCHED OVER TO ACQUIRE 17000 FT; THE AIRSPD BEGAN BUILDING VERY RAPIDLY. I (CAPT) WAS FLYING; SO I PULLED BOTH THROTTLES BACK TO MID POS. I WAS LOOKING AT THE N1 GAUGES AS I DID SO; AND I NOTICED THAT #2 N1 DID NOT REDUCE FROM ITS CLB SETTING OF 91%. THE FO REMARKED THE AIRSPD WAS IN THE 320-330 KT RANGE. I PULLED THE #1 THROTTLE BACK TO KEEP THE AIRSPD IN LIMITS. THE FO TOLD ATC THAT WE HAD A PROB AND WE WANTED TO STAY AT 17000 FT WHILE WE EVALUATED IT. THE FO RADIOED EWR MAINT FOR ASSISTANCE. WE MADE SEVERAL ATTEMPTS TO REGAIN CTL OF THE ENG (TURNED OFF THE PWR MANAGEMENT COMPUTERS; TRIED ENGAGING THE AUTOTHROTTLES; LOOKED FOR CIRCUIT BREAKERS TO PULL; ETC -- ALL WITHOUT RESULT); BUT COULD NOT. I DECLARED AN EMER AND WE REVERSED COURSE FOR EWR. NEITHER THE QRH NOR THE FLT MANUAL HAD ANY PROC FOR THIS PROB. WE REJECTED ANYTHING THAT INVOLVED A RESTART DUE TO THE FACT THAT THE ENG WAS AT SUCH A HIGH PWR SETTING. HAD THE ENG BEEN AT AN INTERMEDIATE PWR SETTING; OR DOWN TOWARDS IDLE; WE MIGHT HAVE TRIED A RESTART. ATC GAVE US A DSCNT CLRNC. WE TRIED DSNDING AT A SHALLOW ANGLE WITH THE SPD BRAKE OUT AND #1 ENG AT IDLE TO SEE IF WE COULD KEEP THE AIRSPD WITHIN LIMITS. IT BECAME APPARENT THAT AS WE DSNDED INTO THE THICKER AIR; THE AIRSPD WOULD SOON BE BUMPING UP AGAINST THE MAX LIMIT. I TOLD THE FO TO OPEN THE QRH TO THE 'ENG FIRE/FAILURE/SEPARATION' CHKLIST AND TO PREPARE TO SHUT THE ENG DOWN SO WE COULD DSND AT A MORE REASONABLE SPD. WE RAN THE CHKLIST; SHUTTING DOWN THE #2 ENG. IT WAS A NOISY; 'AGGRESSIVE' SHUTDOWN; IN THAT THERE WAS A LOT OF YAW. I TOLD THE PAX WHAT HAD HAPPENED AND WHAT WE WERE DOING AND THAT WE WERE HDG BACK TO EWR (I HAD ALREADY TOLD THE LEAD FLT ATTENDANT). WE MADE AN UNEVENTFUL LNDG AT EWR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THERE WAS NO CTL OVER THIS ENG EXCEPT TO SHUT IT DOWN. THE RPTR SAID IT WAS NOT A CASE OF THE THRUST LEVER BEING FROZEN SINCE THE LEVER MOVED SMOOTHLY IN MANUAL AND THE AUTOTHROTTLE SYS MOVED IT FORWARD AND AFT. THE RPTR STATED MAINT DID NOT RPT THE CORRECTIVE ACTION TAKEN BUT SUSPECTS POSSIBLE MAIN FUEL CTL FAILURE.