What was found
DURING MAINTENANCE ENGINE RUNS, NOTED LEFT ENGINE BLEED PRESSURE WAS LOWER THAN THE RIGHT ENGINE AT IDLE POWER SETTINGS. THE LEFT ENGINE BLEED AIR PRESSURE WOULD RISE AND MATCH THE RIGHT PRESSURE ONCE THE POWER LEVERS WERE ADVANCED TO CHANGE THE ENGINE BLEED SOURCE FROM HIGH TO LOW STAGE. USING THE FIM AND A DESCRIPTION OF THE SYSTEM OPERATION, DETERMINED EITHER THE HIGH STAGE REGULATOR OR HIGH STAGE VALVE TO BE AT FAULT. REPLACED THE LEFT HIGH STAGE VALVE. PERFORMED AN ENGINE RUN AND THE LEFT AND RIGHT ENGINE BLEED PRESSURES MATCHED AT IDLE POWER SETTINGS CORRECTING THE PROBLEM.