AN MD80 FUEL SYSTEM MALFUNCTION CAUSED THE R TANK FUEL TO PUMP INTO THE CTR TANK AS THE L TANK DECREASED CAUSING AN IMBALANCE. AN EMER WAS DECLARED AND THE FLT DIVERTED TO A NEARBY ARPT.

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ENRTE FROM ZZZ1 TO ZZZ2 SHORTLY AFTER LEVELING OFF AT CRUISE ALT; WE NOTICED THE FUEL WAS NOT FEEDING CORRECTLY FROM THE FUEL TANKS (NOTE: FUEL WAS FEEDING NORMALLY AT THE AFTER TKOF CHKLIST). THE L MAIN TANK WAS INDICATING APPROX 1000 LBS LESS THAN THE R MAIN TANK AND THE CTR TANK FUEL QUANTITY WAS INCREASING WITHOUT EXPLANATION (NOTE: THE FUEL TOTALIZER INDICATED THE CORRECT TOTAL FUEL QUANTITY AS REFED BY THE FLT PLAN). WE ATTEMPTED THE FUEL XFEED PROC AND IT HAD NO EFFECT ON THE IMBAL. THE QRH PROCS INCLUDING THE 'FUEL LEAK' AND THE 'CTR TANK FAILS TO FEED' PROC ALSO HAD NO EFFECT ON THE IMBAL OR FEED SITUATION. ALL CIRCUIT BREAKERS WERE IN AND THERE WERE NO ABNORMAL INDICATIONS ON THE OAP. IN COORD WITH DISPATCH; I ELECTED TO DIVERT TO ZZZ3 AND DECLARE AN EMER. WE LANDED UNEVENTFULLY AT ZZZ3 WITH A FUEL IMBAL OF APPROX 3400 LBS AND TOTAL FUEL INDICATING APPROX 17300 LBS. 1) APPEARS THAT THERE WAS A MECHANICAL FAILURE IN THE FUEL SYS. 2) CREW COORD BTWN THE FO AND I AND THE FLT ATTENDANTS WORKED VERY WELL. 3) ZZZ CTR ATC WAS VERY HELPFUL IN EXPEDITING OUR DIVERT TO ZZZ3.