What was found
A THIRTY DEGREE SPLIT IN ITT WAS REPORTED WITH LEFT ENGINE RUNNING HIGHER THAN RIGHT. A POWER ASSURANCE RUN VERIFIED A PROBLEM WITH THE LEFT BLEED AIR CONTROL SYSTEM. FUNCTIONAL CHECKS OF THE LEFT BLEED SYSTEM WERE UNRELIABLE AND DID NOT AGREE WITH TROUBLESHOOTING NARRATIVES IN THE BEECH 1900D MM. A VISUAL INSPECTION REVEALED THE LEFT BLEED AIR DUCT WAS BROKEN AT THE TEMPERATURE CONTROL BOSS. THE DUCT WAS REPLACED WITH A NEW UNIT.