What was found
NOXIOUS VAPOR IN CABIN. FLAP CIRCUIT BREAKER POPPED. THE PILOT NOTED AN UNKNOWN AIRBORNE IRRITANT IN THE COCKPIT WHICH RESULTED IN A BURNING SENSATION IN THE EYES, NOSE AND THROAT. THE PIC DECLARED AN EMERGENCY AND RETURNED TO EWR AND LANDED SAFELY. MX: 1) PERFORMED ENGINE COMPARTMENT INSPECTION IAW C208 AMM 71-00-01. NO DEFECT NOTED. PERFORMED ENGINE RUN IAW MANUFACTURE. PILOTS OPERATING HANDBOOK. NO NOXIOUS FUMES OR OTHER SMELLS DETECTED. LUBRICATED FLAP BELL CRANK IAW C208-MM-12-21-02. FLAP OPERATED FREE AND CORRECT. PERFORMED OPS CHECK ON FLAP NORMAL AND STBY FLAP SYSTEM, CYCLED BOTH NORMAL AND STBY SYSTEM INDIVIDUALLY, IAW MM-27-50-00, ALL FLAP POSITION CYCLED WITHOUT POPPING FLAP CB.