PERSISTENT ODORS; FUMES AND THE ACTIVATION OF A SMOKE ALARM IN THE AFT CABIN RESULTS IN AN OVERWEIGHT EMERGENCY RETURN TO THE DEP STATION FOR A B757-200.

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UPON ARRIVING AT THE ACFT; AN EXTREMELY FOUL ODOR WAS NOTED IN THE CABIN. THE LOGBOOK HAD 4 PREVIOUS WRITE-UPS DEALING WITH THIS PROB. THE MOST RECENT WRITE-UP WAS ON THE PREVIOUS FLT. THE SIGNOFF STATED THAT THE AIR CONDITIONING SYS HAD BEEN CHKED AND CLEANING HAD BEEN PERFORMED AND THE ACFT WAS ODOR FREE (THIS IS A PARAPHRASE). SINCE THE ACFT WAS OBVIOUSLY FOUL SMELLING; I REQUESTED THAT MAINT COME TO THE ACFT. MY REQUEST WAS RESISTED WITH THE COMMENT THAT 'THE ACFT HAS BEEN CLOSED UP IN THE HANGAR ALL DAY' AND THAT WAS WHY IT SMELLED. I INSISTED THAT I NEEDED A TECHNICIAN AT THE AIRPLANE TO DISCUSS ITS CONDITION. THE TECHNICIAN AGREED THAT THE AIRPLANE SMELLED PRETTY BAD; BUT THAT AFTER ENG START; THE SMELL WOULD DISSIPATE. I AGREED TO TAKE THE ACFT AND THE SMELL DID LESSEN AFTER ENG START. PRIOR TO TKOF; I CHKED WITH THE PURSER; AND SHE AGREED THAT THE ODOR WAS GOING AWAY. WE TOOK OFF AND HAD AN UNEVENTFUL CLBOUT TO FL360. THE SMELL WAS PERSISTING TO SOME EXTENT; AND TO HELP VENTILATE THE CABIN; I TURNED OFF THE R RECIRCULATION FAN. SHORTLY AFTER THAT; A FLT ATTENDANT CALLED TO RPT THAT SHE SMELLED 'ELECTRICAL SMOKE.' I SENT THE FO BACK TO VERIFY THE FLT ATTENDANT'S PERCEPTIONS. AS HE WAS HEADING BACK; A SMOKE ALARM IN THE AFT OF THE ACFT STARTED GOING OFF. AT THIS POINT; I TURNED THE RECIRCULATION FAN BACK ON. WHEN THE FO RETURNED; HE STATED THAT HE REALLY DID NOT SEE OR SMELL ANYTHING; BUT HE HAD NOT GONE TO THE REAR OF THE CABIN. I HAD DECIDED TO RETURN TO ZZZ AND WE COORD WITH ZZZ CTR. THE FUMES WERE LESSENING IN THE CABIN; AND THE SMOKE ALARM STOPPED. AS WE GOT CLOSER TO ZZZ; WE DECLARED AN EMER DUE TO OUR OVERWT LNDG CONDITION. THE TOUCHDOWN WAS SMOOTH AND MINIMAL BRAKING WAS USED. WE PARKED AT THE GATE. AFTERWARDS; IN TALKING TO PEOPLE THAT WERE IN THE REAR OF THE CABIN; THEY STATED THAT THERE WAS A DEFINITE VISIBLE HAZE IN THE CABIN WHEN THE ALARM WAS GOING OFF.