MD88 FLT CREW NOTICES APPARENT FUEL LOSS FROM L WING TANK AND AFTER TROUBLESHOOTING ELECTS TO DIVERT TO NEAREST SUITABLE ARPT.

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JUST PRIOR TO LEVELOFF AT 33000 FT; THE FO NOTICED THAT WE WERE BURNING FUEL OUT OF OUR L MAIN TANK AT A HIGH RATE. WE SWITCHED THE FUEL PANEL SELECTOR FROM B TO A AND INDICATED THE SAME PROB. WE RAN THE QRH CHKLIST 'CTR TANK FAILS TO FEED INFLT.' AS INSTRUCTED WE SELECTED BOTH L MAIN FUEL PUMPS TO OFF AND OBSERVED THAT THE L MAIN TANK WAS DECREASING AT A SLOWER RATE; AND THAT THE CTR TANK FUEL WAS INCREASING. WE HAD RULED OUT A FUEL LEAK SINCE THE FUEL AT TOP OF CLB WAS ABOVE FLT PLAN BURN. XFEEDING WAS RULED OUT BECAUSE WE COULD NOT DETERMINE THE CAUSE OF THE PROB. ALL FUEL PUMPS APPEARED TO BE WORKING NORMALLY. AT THIS POINT WE HAD A FUEL STATUS OF 5500 LBS IN THE L MAIN TANK; 9200 LBS IN THE R MAIN; AND 12300 LBS IN THE CTR. WE THEN BOTH DECIDED THAT IT WAS TIME TO LAND AT THE NEAREST SUITABLE ARPT. WE NOTIFIED ATC AND REQUESTED VECTORS TO DEVIATE TO ZZZ DUE TO A FUEL PUMP AND FUEL DISTRIBUTION PROB. I SEL CALLED DISPATCH AND MAINT CTL AND TRIED TO COMMUNICATE THE PROB. THE SEL PATCH WAS VERY BROKEN UP AND SOMEWHAT UNREADABLE; BUT WAS ABLE TO LET THEM KNOW THAT WE WERE DIVERTING TO ZZZ AND THAT WE HAD DECLARED AN EMER. I CALLED THE #1 FLT ATTENDANT AND EXPLAINED TO HER THAT WE HAD A FUEL DISTRIBUTION PROB AND WILL HAVE TO DIVERT TO ZZZ. I HAD ASKED HER TO STOP ALL SVC; STOW ALL EQUIP AND PREPARE FOR A NORMAL APCH AND LNDG. IF ANYTHING WAS TO CHANGE; I WOULD LET HER KNOW. I THEN MADE A SIMILAR STATEMENT TO THE PAX AND EXPLAINED THAT WE COULD NOT CONTINUE UNTIL WE COULD FIX THE PROB AND WE WILL BE MAKING A DSCNT AND LNDG IN ZZZ. AS WE WERE BEING VECTORED; I ELECTED TO DISCONNECT THE AUTOPLT AND HAND FLEW THE ACFT AT STRAIGHT AND LEVEL FLT TO TEST FOR ANY UNUSUAL FLT CHARACTERISTICS DUE TO THE UNBAL OF FUEL. THERE WERE NONE. THE ACFT FLEW STRAIGHT AND LEVEL HANDS OFF. WE BEGAN A DSCNT AND LNDG IN ZZZ. THE LNDG AND TAXI TO THE GATE WERE UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINT DISCOVERED THAT A FUEL LINE FROM THE L WING TANK HAD SEPARATED FROM THE FUEL MANIFOLD LOCATED INSIDE THE CTR FUEL TANK ALLOWING FUEL TO XFER FROM THE L WING TANK UNCONTROLLABLY. HOW THIS FAILURE OCCURRED WAS NOT REVEALED TO THE RPTR.