What was found
500 LBS OF FUEL MIGRATED BACK INTO CENTER FUEL TANK AFTER 4 HRS OF FLT TIME. DEBRIEFED CAPT & F/O AND BOTH BELIEVED FUEL IS TRANSFERRING FROM LEFT TANK TO CTR TANK DUE LEFT FUEL USAGE IS ABOUT 50 LBS AN HOUR GREATER. THEY BURNED THE 500 LBS TO 0 BUT ONE HOUR LATER, IT INCREASED TO 80 LBS. ACP FUNCTIONAL TEST OF THE CENTER TANK FUEL PUMP CANISTER CHECK VALVES, VALVES CHECK GOOD NO LEAKAGE. A-C OK TO CONTINUE IN FLIGHT PER MM 28-21-00-810-840-A PAGE 3 ITEM A.3. DUE TO NO PRIOR HISTORY.