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 HIGH STAGE REGULATOR AND NO PRESSURE INCREASE OCCURRED AT IDLE. SEE SDR COZA20161217101 FOR REPLACEMENT OF THE HIGH STAGE VALVE.