What was found
RIGHT ENGINE FIRE WARNING ILLUMINATED ABOVE FL400. AFM PROCEDURES FOLLOWED AND WARNING CEASED. WARNING DID NOT ILLUMINATED BELOW FL380. MAINTENANCE FOUND THE LEFT BLEED MIXING VALVE STUCK IN LOW CAUSING THE RIGHT SIDE TO OVERHEAT THE FIRE DETECTION LOOP. LEFT MIXING VALVE REPLACED WITH OVERHAULED UNIT. GROUND TESTING SATISFACTORY. MAINTENANCE CHECK FLIGHT CONDUCTED. R/H FIRE WARNING AT ALTITUDE OCCURRED ONLY WITH R/H BLEED SWITCH ON. R/H ENGINE REMOVED, ALL ACCESSIBLE BLEED SYSTEM SEALS REPLACED AND HOT SECTION FIRE LOOP REPLACED. FOLLOW-UP CHECK FLIGHT SHOWED NO FIRE WARNING.