What was found
AFTER STARTING ENGINE 2 (FOR SINGLE ENGINE TAXI) TURNED PACKS ON AND STARTED TO HEAR A WHIRRING NOISE. SUBSEQUENTLY, SECONDS LATER WHAT WOULD APPEAR TO BE SMOKE WAS ENTERING THE COCKPIT WITH A BAD AROMA.IMMEDIATELY TURNED PACKS OFF.STARTED ENGINE 1 AND PULLED BACK INTO THE GATE. (FLIGHT 5526 - FILED CLOSED)RAN BOTH PACKS INDIVIDUALLY FOR APPROX 30 MINUTES. NOTED FOUL SMELL IN FLIGHT DECK ONLY WHEN L PACK WAS ON. VERIFIED L PACK PRESSURE WAS HIGHER THAN RIGHT PACK BY 25PSI. ECS SYNOPTIC ALSO SHOWED L PACK TEMP SURGED TO 105C. L PACK PRESS & L PACK HIGH TEMP INTERMITTENTLY DISPLAY ON EICAS. INSPECTED BOTH PACKS DUCT INTEGRITY; NO DEFECTS WERE NOTED. CHECKED WATER SEPARATOR DRAIN LINES; NO CLOG NOTED. CHECKED OIL & OBSERVED NO OIL ON L ACM SIGHT GLASS. PLEASE REF. AML #1639597 FOR L PACK DEFERRAL.