What was found
AFTER STARTING NR 1 ENGINE AND TURNING ON THE NR 1 PACK, CREW REPORTED A VERY STRONG ODOR FILLED THE COCKPIT - DIRTY SOCK SMELL. IRRITATING TO EYES AND LUNGS. TURNED PACKS OFF AND RETURNED TO GATE. MAINTENANCE ACCOMPLISHED J/C 03L00021001, "737 T/S CABIN ODOR". BOROSCOPED LEFT AND RIGHT ENGINES, AS PRECAUTIONARY HPC 1-4 STAGES, WITH NO FINDINGS. REPLACED LEFT AND RIGHT WATER SEPARATOR BAGS. ACCOMPLISHED J/C 03L00036001, "BLEED AIR ANALYSIS". SENT FILTERS TO PIT FOR ANALYSIS. PERFORMED PACK BURN-OUT PER AMM 21-00-01-102-001.