INFLT FIRE WARNING. EMER DECLARED.

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WE DEPARTED FROM MDW ON RWY 31C AT A TKOF WT OF 85521 LBS. THE CONDITIONS DURING DEP WERE ESTIMATED CEILING 1700 FT OVCST; 12 MI VISIBILITY; TEMP 30 DEGS; DEWPOINT 28 DEGS; WINDS WERE 330 DEGS AT 15 KTS. WE HAD TURNED ON THE ENG HEAT DURING THE START PROCS. WE TURNED ON THE AIRFOIL ICE PROTECTION DURING THE CLB; AS WE ENTERED THE CLOUDS. I CALLED FOR 'CLB PWR; CLB CHKLIST' AT 3000 FT AGL; AND SET BOTH ENGS AT 1.80 EPR (RAT/EPR GAUGE CALLED FOR 1.93 CLB PWR). ABOUT 4000 FT MSL; L ENG EPR DROPPED TO 1.6; AND AFTER APPROX 10 SECONDS; THE L ENG FIRE WARNING SOUNDED WITH ALL ASSOCIATED LIGHTS. I WAS THE PF; SO I RETARDED L THROTTLE BACK TO IDLE; AND THE FIRE WARNING WENT OUT IMMEDIATELY. WE SUSPECTED BLEED AIR LEAK AT THIS TIME; AND ASKED ATC TO MAINTAIN HDG TO CHK INTO SOME INDICATIONS. WE WERE ABLE TO LEVELOFF AT 6000 FT; ABOVE THE CLOUDS; AND TURNED OFF ALL ICE PROTECTION; AND CLOSED THE XFEED VALVES. WHEN THE AIRFOIL ICE PROTECTION CYCLE COMPLETED ITSELF; I INCREASED PWR ON THE L ENG; AND AT 1.4 EPR; THE FIRE WARNING SIGNALS REAPPEARED. I RETARDED THE PWR TO IDLE; THE SIGNALS WENT AWAY. WE DECLARED AN EMER; AND REQUESTED AN IMMEDIATE RETURN TO MDW (DUE TO SIGNS OF HOT AIR LEAK) FROM ATC. WE CONFIRMED THE WX IN MDW; NOTIFIED THE FLT ATTENDANTS; THEN WE TOLD THE PAX THAT WE WERE RETURNING TO MDW AS A PRECAUTIONARY MEASURE DUE TO SIGNS OF HOT AIR LEAK FROM L ENG. WE COMPLETED THE DSCNT CHKLIST; I BRIEFED THE ILS 31C AT MDW; AND WE DSNDED TO 2500 FT MSL. AT APPROX 2600 FT MSL; BELOW THE ICING; I CHOSE TO SHUT DOWN THE L ENG TO STOP THE HOT AIR LEAK. WE COMPLETED THE INFLT ENG SHUTDOWN CHKLIST AND WE WERE CLRED FOR THE ILS RWY 31C AT MDW. I FLEW THE APCH; AND WE COMPLETED THE BEFORE LNDG; AND 1 ENG BEFORE LNDG CHKLIST. WE HAD ASKED FOR THE GND EQUIP TO BE STANDING BY; JUST TO CONFIRM THERE WAS NO SMOKE ON THE L SIDE OF THE ACFT. WE LANDED; TURNED OFF CLR OF THE RWY WITHOUT INCIDENT; WHERE THE TRUCKS CAME UP TO THE REAR OF THE AIRPLANE; AND CONFIRMED THERE WAS NO SMOKE. ALL PROCS AND CHKLISTS WERE COMPLETED AT THE PROPER TIMES WITH COMPLIANCE WITH COMPANY POLICIES. I WOULD LIKE TO ADD THAT MY ENTIRE CREW HANDLED ITSELF IN A PROFESSIONAL AND CALM MANNER AT ALL TIMES.