What was found
DURING T/O, FOLLOWING GEAR UP TRANSITION THE CREW FELT A LARGE BUMP FROM THE LT SIDE OF THE AC. AT APPROX 4,000 FT A PASSENGER REPORTED SEEING SOMETHING HANGING FROM JUST BEHIND THE PROP, WHICH WAS CONFIRMED BY THE F/O UPON FURTHER INVESTIGATION. THE CREW ALSO NOTICED A TORQUE INCREASE TO JUST OVER 100 PERCENT ON THE LT ENGINE. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED, LANDING SAFELY. MAINT FOUND THE LT AFT ENGINE OIL COOLER DOOR OPEN, REMOVED AND REPLACED IT. MAINT PERFORMED ENGINE GROUND RUNS IAW MM. ALL OPS CHECK GOOD. AC OK FOR SERVICE. (K)