Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 1312; Flight Crew Type 1189
Situational Awareness
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Aircraft
We left on time and had some weather to deal with on the way out. I check fuel balance upon going through FL180; as per recommend in the FM. There was a slight imbalance of .2.  Which was normal. We burned through the center tank fuel and I turned them off and rechecked the quantities. Same split. .2. After 10-20 minutes the Fuel Imbalance message came on the EICAS. It showed left tank at 14.3 and right tank at 12.3. That was alarming. I assumed a fuel leak because we didn't cross feed at any point on the flight. We watched it and ran the QRH up to the engine shut down. The QRH confirmed a suspected fuel leak. You could count the fuel leaving the right tank. The totalizer and fuel indicator both agreed on total amount on board-that was just a huge split in the wings so it was not a sensor issue. The CA immediately contacted dispatch for available places. Dispatch came back with ZZZ1. The flight attendants did check the wing and could not see any fuel leaking. We advised ATC and headed straight to ZZZ1. We were roughly 80 miles from the airport. The CA and I agreed to not shut down the engine since the fuel was leaking slowly. We kept the number 2 engine at idle and landed with two engines. We did check the roll and yaw tendency and it was noticeable but not uncontrollable before we landed- due to fuel split. Upon arriving in ZZZ1 the fire department could not find a leak and maintenance was trouble shooting for a couple hours and agreed there was no sensor problem we somehow were losing gas out of the right side. They weren't sure from were. We land and the right side was down to 9.8 and the left was at 13.7. We deplaned as directed a few rows at a time for unbalance problems that might occur and none did. Dispatch did come back and requested we continue to ZZZ- we agreed that was not a good idea. ZZZ1 was closer and we did not know the extent of the fuel leak - it seemed slow but we could not determine if it was going to get worse so we declined going to ZZZ and continued to our nearest suitable airport of ZZZ1.
B757 First Officer reported a possible fuel leak at FL340 when EICAS indicated a fuel imbalance. The crew elected to divert to the nearest suitable airport and landed with a 4;000 pound split between the right tank and the left.