A DC9-30 ON INITIAL CLB DECLARED AN EMER AND DIVERTED DUE TO A L ENG FIRE WARNING CAUSED BY AN ENG PNEUMATIC LEAK.

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AFTER DEPARTING RWY 4 AT LGA AFTER GEAR UP AND PASSING 1000 FT AGL; L ENG (AND FIRE BELL) AND FIRE LIGHT ILLUMINATED. I AND CAPT VERIFIED FIRE INDICATION WITH BOTH A AND B LOOPS ILLUMINATED. EMER CHKLIST ACCOMPLISHED. AFTER RETARDING L THROTTLE TO IDLE (PART OF EMER PROC) FIRE INDICATIONS CEASED. DECLARED EMER AND REQUESTED VECTORS BACK TO LGA FOR FULL STOP LNDG. FLT ATTENDANTS NOTIFIED OF NATURE OF EMER. UNEVENTFUL APCH AND LNDG (FLAPS 25 DEGS) TO RWY 4 AT LGA. UPON CLRING RWY FIRE RESCUE MET US AND REQUESTED US TO SHUT DOWN L ENG TO INSPECT. NO PROBS WITH ENG FOUND. HAD TO BE TOWED BACK TO ACR GATE. (ACFT WOULD NOT TAXI AT THAT WT; 97800 LBS ON 1 ENG.) NOT SURE WHAT CAUSED THE PROB; BUT DUE TO ALL FIRE INDICATIONS GOING OUT AFTER RETARDING THROTTLE TO IDLE; WOULD SUSPECT BLEED AIR LEAK. (WE TESTED FIRE DETECTION SYS AFTER LIGHTS WENT OUT AND GOT A GOOD CHK.) UPON TALKING WITH LGA ACR MAINT AT LGA; THEY SUSPECTED A BLEED AIR LEAK AS WELL; THOUGH WE NEVER FOUND OUT WITH MAINT AS TO FINAL DETERMINATION.