B777 FLT CREW HAS A FUEL SYS MALFUNCTION WHILE ENRTE.

Narrative

WHILE ACTING IN THE CAPACITY OF RELIEF PLT (PNF) AND OCCUPYING THE FO SEAT IN CRUISE FLT AT FL340 E OF YEV; THE FUEL IMBAL EICAS ILLUMINATED AND ALERTED US OF AN IMBAL BTWN FUEL TANK #1 AND FUEL TANK #2. THE FUEL QUANTITY IN FUEL TANK #1 WAS ABOUT 4000 LBS LOWER THAN THE FUEL TANK #2. THE OTHER RELIEF PLT (PF) AND I RAN THE CORRESPONDING FUEL IMBAL CHKLIST; CHKING ALL PARAMETERS PER THE CHKLIST. THE FUEL CONTINUED TO DECREASE IN THE L TANK AT AN ABNORMAL RATE AND WE CALLED THE CAPT TO THE COCKPIT AND INITIATED THE FUEL LEAK CHKLIST. COM WAS ESTABLISHED WITH MAINT DISPATCH AND THE B777 ASSISTANT CHIEF PLT. WE ALSO ATTEMPTED TO CONTACT BOEING B777 TECHNICAL SUPPORT; WHICH WAS UNSUCCESSFUL. DURING THE TROUBLESHOOTING STAGE OF THE FLT; WE DISENGAGED THE AUTOPLT AND DETERMINED THAT THE ACFT WAS IN TRIM; WITH NO ADVERSE ROLL. THE FUEL TANK #1 GAUGE INDICATED A FUEL IMBAL OF MORE THAN 40000 LBS FROM THE FUEL TANK #2 GAUGE AND WAS CONTINUING TO DECREASE. WE ALSO PULLED UP THE FUEL SYS MAINT PAGE ON THE FMC TO CHK THE FUEL PROBE READING AND FOUND THAT ONE OF THE PROBES DID NOT READ CLOSE TO ANY READING IN THE OPPOSITE #2 TANK OR ITS OWN TANK. THIS HELPED THE CAPT TO DECIDE THAT IT WAS A FUEL GAUGE MALFUNCTION AND NOT A FUEL LEAK. AFTER ASSESSING THE SITUATION WITH ALL THE PARTICIPANTS; THE DECISION WAS MADE BY THE CAPT TO DIVERT TO ANC. IN THE DSCNT THE #1 TANK FUEL GAUGE STARTED TO INCREASE FUEL QUANTITY; WHICH HELPED TO REINFORCE OUR DECISION. WE MADE A SMOOTH OVERWT LNDG IN ANC.