What was found
L & R DUCT PRESSURE DIFFERENTIAL OUT OF LIMITS DURING ENTIRE CLIMB TO FL310. LEFT DUCT READ AS HIGH AS 65 PSI BUT FLUCTUATED BETWEEN 58 AND 65 DURING CLIMB, RIGHT DUCT WAS STEADY AT 37 PSI. AFTER LEVEL OFF 310, LEFT DUCT DECREASED TO WITH IN LIMITS TO 47 PSI AND RIGHT WAS STILL 37 PSI. GROUNDED - YES. DURING TROUBLE SHOOTING FOUND #1 ENGINE PRSOV NOT FULLY CLOSED REF B737-900 AMM 36-11-00-700-804 FOR HEALTH CHECK AND 36-00-00-860-807 PRSOV DOES NOT MOVE SMOOTHLY. REMOVED AND REPLACED PRSOV ON #1 ENGINE. PERFORMED OPERATIONAL CHECK AND FOUND HOSE ASSEMBLY TO PRE-COOLER LEAKING. OPS CHECK OF PRSOV WAS GOOD REF AMM 36-11-04-400-801. REMOVED AND REPLACED HOSE ASSY P/N 16135-81 TO PRE-COOLER ENGINE RUN OPERATIONAL CHECK GOOD. NOTED DUCT PRESSURE WAS EVEN BETWEEN L AND R WITH ENGINES AT 85 N2.