What was found
MDW - FLT 575 - AT BEGINNING OF TAKEOFF ROLL WHEN POWER APPLIED, COCKPIT FILLED WITH HEAVY SMOKE. TAKEOFF ABORTED, RETURNED TO GATE FOR MX ACTION. REMOVED AND REPLACED FAN MOD. VALVE IAW MM 36-11-16, PERFORMED OPERATIONAL CHECK TO THE AIR SUPPLY DISTRIBUTION SYSTEM IAW 36-11-00. RAN BOTH ENGINES TO TAKEOFF POWER. OPS CHECK GOOD, NO SMOKE NOTED.