Narrative:

We were scheduled to operate a revenue flight from ZZZ to ZZZ1. We arrived at the aircraft approximately 40 minutes prior to block and began our preflight. While accomplishing the cockpit preflight the fueler came into the cockpit to see if we were good with the current fuel load. At this time we noted that the aircraft had more than the planned departure fuel since the aircraft was originally scheduled as a hot spare. My planned total fuel was 26.7 and our current total was 29.7. The fueler asked if we were okay with the current fuel imbalance which was only 400 pounds; left side being 400 pounds less that the right. We noted that our center tank contained 1;500 pounds and that we would have the 'fuel config' EICAS message during our initial phases of operations. Stating we had more than our planned fuel and the current imbalance wasn't significant enough to be a concern; he departed. Approximately 30 minutes before scheduled departure we turned on the APU and continued our preflight duties. The sort was running late and we didn't block out until 29 minutes late. Our APU had been on for approximately 1 hour; possibly leading to an extra 200 pounds imbalance on the left side.I was the flying pilot for this leg. Start up; taxi out; takeoff and initial climb out were completely normal with no irregularities noted. Passing through 10; 000 feet I called for the first officer to turn on the left center fuel pump to begin fixing our fuel imbalance. At this time we noticed that there was now a 2300 pound split between the left and right fuel quantity indicators where there had only been a 400 pound imbalance earlier. Concerned that we now had exceeded the 1800 pound imbalance limit for the B757 I called for him to open the fuel crossed valve and secure the left engine fuel pumps to help expedite the fuel transfer. At this time the first officer noted that the fuel quantity in the left tank indicator was falling and that we looked like we were losing fuel. I called for the autopilot and began watching the quantity indicator. Our recollection was that we saw the left fuel quantity go from 12.0 to 11.8 in less than 1 minute. Fearing a significant fuel leak; I called for the first officer to run the 'fuel leak engine' checklist. In accordance with the checklist; we began diagnosing the problem. We noted that of the 7 possible indicators of a potential fuel leak; we had 2 (EICAS fuel config and observed left tank quantity falling at an excessive rate). Continuing with the checklist we secured the center pump switches and fuel crossfeed switches. At this point I do not recall how much increase in the difference there continued to be between the left and right fuel quantity indicators; but we had at least a 2;300 pounds imbalance between the two tanks in less than 30 minutes. Based on this observation; we felt we had a condition the checklist states should be considered an engine fuel leak.leveling off at FL180 we requested a turn back toward ZZZ and a descent. The first officer continued with the checklist while I sent an ACARS message to dispatch that we were diverting back to ZZZ for a mechanical. In accordance with the checklist we secured the left engine; which was the side we suspected of having a fuel leak; and the first officer [advised ATC]. We continued our descent and received vectors for the ILS to xxr. We reviewed the 'fuel leak engine' checklist and set up for a single engine approach. We accomplished the 'approach checklist' and 'landing checklist' and landed without further incident. We had crash fire rescue equipment (crash fire rescue) inspect the engine while we were stopped on the runway and told that we didn't appear to have any leakage at that time. We then received clearance to taxi clear of the runway and into [the] ramp. We blocked in and executed the 'shutdown and secure checklist' without problems. To the best of my recollection we still had a 600-800 pounds imbalance on shutdown even after operating single engine for at least 20minutes.

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Original NASA ASRS Text

Title: B757-200 flight crew reported fuel leak shortly after takeoff.

Narrative: We were scheduled to operate a revenue flight from ZZZ to ZZZ1. We arrived at the aircraft approximately 40 minutes prior to block and began our preflight. While accomplishing the cockpit preflight the fueler came into the cockpit to see if we were good with the current fuel load. At this time we noted that the aircraft had more than the planned departure fuel since the aircraft was originally scheduled as a hot spare. My planned total fuel was 26.7 and our current total was 29.7. The fueler asked if we were okay with the current fuel imbalance which was only 400 pounds; left side being 400 pounds less that the right. We noted that our center tank contained 1;500 pounds and that we would have the 'Fuel Config' EICAS message during our initial phases of operations. Stating we had more than our planned fuel and the current imbalance wasn't significant enough to be a concern; he departed. Approximately 30 minutes before scheduled departure we turned on the APU and continued our preflight duties. The sort was running late and we didn't block out until 29 minutes late. Our APU had been on for approximately 1 hour; possibly leading to an extra 200 pounds imbalance on the left side.I was the flying pilot for this leg. Start up; taxi out; takeoff and initial climb out were completely normal with no irregularities noted. Passing through 10; 000 feet I called for the First Officer to turn on the Left Center fuel pump to begin fixing our fuel imbalance. At this time we noticed that there was now a 2300 pound split between the Left and Right fuel quantity indicators where there had only been a 400 pound imbalance earlier. Concerned that we now had exceeded the 1800 pound imbalance limit for the B757 I called for him to open the fuel crossed valve and secure the left engine fuel pumps to help expedite the fuel transfer. At this time the First Officer noted that the fuel quantity in the left tank indicator was falling and that we looked like we were losing fuel. I called for the autopilot and began watching the quantity indicator. Our recollection was that we saw the left fuel quantity go from 12.0 to 11.8 in less than 1 minute. Fearing a significant fuel leak; I called for the First Officer to run the 'Fuel Leak Engine' checklist. In accordance with the checklist; we began diagnosing the problem. We noted that of the 7 possible indicators of a potential fuel leak; we had 2 (EICAS Fuel Config and observed left tank quantity falling at an excessive rate). Continuing with the checklist we secured the center pump switches and fuel crossfeed switches. At this point I do not recall how much increase in the difference there continued to be between the Left and Right fuel quantity indicators; but we had at least a 2;300 pounds imbalance between the two tanks in less than 30 minutes. Based on this observation; we felt we had a condition the checklist states should be considered an engine fuel leak.Leveling off at FL180 we requested a turn back toward ZZZ and a descent. The First Officer continued with the checklist while I sent an ACARS message to Dispatch that we were diverting back to ZZZ for a mechanical. In accordance with the checklist we secured the left engine; which was the side we suspected of having a fuel leak; and the First Officer [advised ATC]. We continued our descent and received vectors for the ILS to XXR. We reviewed the 'Fuel Leak Engine' checklist and set up for a single engine approach. We accomplished the 'Approach Checklist' and 'Landing Checklist' and landed without further incident. We had CFR (Crash Fire Rescue) inspect the engine while we were stopped on the runway and told that we didn't appear to have any leakage at that time. We then received clearance to taxi clear of the runway and into [the] ramp. We blocked in and executed the 'Shutdown and Secure Checklist' without problems. To the best of my recollection we still had a 600-800 pounds imbalance on shutdown even after operating single engine for at least 20minutes.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.