What was found
STRONG FUEL SMELL NOTED IN FLT DECK AFTER BOTH ENGINES STARTED AFTER SHUTDOWN FROM RETURN TO GATE NOTED A SLIGHTLY STRONGER FUEL ODOR FROM E1 THAN E2. OPENED FAN AND REVERSER COWLS REF AMM 71-11-04/78-31-00 TO INSPECT E1, NO DEFECTS NOTED, CLOSED ACCESS. INSPECTED BOTH PACK BAYS AND NOTED MILD FUEL ODOR LINGERING. FOUND CENTER TANK POPPET VALVES LEAKING INTO PACK BAY WHEN ACTUATED INSTEAD OF THROUGH FLUID LINE, PERFORMED TAXI OF AIRCRAFT FROM HANGAR TO REMOTE, WHILE WAITING FOR CLEARANCE TO TAXI, MILD FUEL ODOR BECAME PRESENT WITH BOTH ENGINE BLEEDS AND BOTH PACKS SELECTED ON, APU BLEED OFF. AFTER 10 MINUTES ODOR HAD SUBSIDED. AFTER REPLACEMENT OF CENTER TANK SUMP VALVES, RAN APU AND BOTH PACKS FULL HOT WITH NO ODOR. IDLED BOTH ENGINES ONE AT A TIME, APU BLEED OFF, APPLICABLE ENGINE BLEED ON WHEN ENGINE IDLED PACKS NO ODOR. TOOK AIRCRAFT FOR A TAXI AND RAN BOTH ENGINES AT CLIMB POWER FOR 2 MINUTES THEN DOWN TO CRUISE, APU BLEED OFF BOTH PACKS ON NO ODOR IN FLIGHT DECK OR CABIN. AFTER A COOL DOWN REPEATED CLIMB AND CRUISE TEST NO ODOR DETECTED REF AMM 71-00-00.