Narrative:

We were given [aircraft] in dsm which had fan blades replaced on the left engine and a subsequent engine run to check for vibrations. Fast forward to climb out through the transition altitude. I was the PF (pilot flying) and when the captain reached up to turn off the lights passing 18;000 ft.; he noticed an approximately 2000 lb. Imbalance between the right and left engines (the left engine had approximately 12.7 lb. And the right engine had approximately 14.6 lb.). Shortly after that the fuel configuration light illuminated. He directed me to continue flying while he got out the QRH. We both watched the left fuel tank quantity decrease by about 100 lb. Every 15 seconds (wings level climb). This combined with the fact that we knew they had worked on the left engine added to the belief that we had a serious fuel leak somewhere in the engine. Adding to the confusion; we were flying an empty aircraft and had the 15;000 lb. Of ballast fuel. This means that during the preflight we had to subtract 15k from the fob in the FMS as it was unusable and accounted for in the ZFW. In the heat of the moment believing we were losing fuel; the captain compared totalizer to prog/2 and noted a vast discrepancy which didn't make sense (the totalizer had a higher value than the calculated which should have clued us in again... If we had added the 15k of ballast fuel to the calculated value it would have been close to the totalizer value). At this point we were about 29;000 ft. And believing we had a significant fuel leak we felt it most prudent to get the jet pointed at the nearest suitable airfield which was dsm. The captain coordinated with ATC and [requested priority handling] while I continued to fly. He referenced the fuel leak checklist in the QRH and as we were descending through about 24;000 ft. Already heading back to dsm; step 3 of the fuel leak checklist directed us to check the cross feed valve. The captain closed the cross feed valve and informed me of the mistake. We discussed whether or not we had enough fuel to continue on to ZZZ or to return to dsm and agreed that the safer option was to continue to dsm to get more fuel before proceeding to ZZZ. The captain terminated the emergency with ATC at this point. Back in the chocks we informed dsm personnel and the fire chief of the error and that there was nothing wrong with the aircraft. We coordinated with dsm personnel as well as our dispatcher to get the flight headed back to ZZZ. The captain made a phone call to the duty officer to inform him of the scenario.in hindsight; here is a list of contributing factors (not intended to be excuses; just an analysis of the occurrence):1. Maintenance leaving switches in abnormal positions than we are used to2. Fact that the fuel quantity was rapidly decreasing on the left side which was the side they had worked on3. Crossflow switch 'shutter' flow bar was faded (we compared to the flow bars in the fuel pump switches and there was a difference; took picture of this after parked.4. Ballast fuel modification to FMS when comparing totalizer to calculated on prog/2 cause:1. Maintenance leaving switches in abnormal positions than we are used to2. Fact that the fuel quantity was rapidly decreasing on the left side which was the side they had worked on3. Crossflow switch 'shutter' flow bar was faded (we compared to the flow bars in the fuel pump switches and there was a difference; took picture of this after parked.4. Ballast fuel modification to FMS when comparing totalizer to calculated on prog/2

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Original NASA ASRS Text

Title: B757 flight crew reported fuel imbalance due to improper fuel system operation.

Narrative: We were given [Aircraft] in DSM which had fan blades replaced on the left engine and a subsequent engine run to check for vibrations. Fast forward to climb out through the transition altitude. I was the PF (Pilot Flying) and when the Captain reached up to turn off the lights passing 18;000 ft.; he noticed an approximately 2000 lb. imbalance between the right and left engines (the left engine had approximately 12.7 lb. and the right engine had approximately 14.6 lb.). Shortly after that the fuel configuration light illuminated. He directed me to continue flying while he got out the QRH. We both watched the left fuel tank quantity decrease by about 100 lb. every 15 seconds (wings level climb). This combined with the fact that we knew they had worked on the left engine added to the belief that we had a serious fuel leak somewhere in the engine. Adding to the confusion; we were flying an empty aircraft and had the 15;000 lb. of ballast fuel. This means that during the preflight we had to subtract 15k from the FOB in the FMS as it was unusable and accounted for in the ZFW. In the heat of the moment believing we were losing fuel; the Captain compared totalizer to PROG/2 and noted a vast discrepancy which didn't make sense (The totalizer had a higher value than the calculated which should have clued us in again... If we had added the 15k of ballast fuel to the calculated value it would have been close to the totalizer value). At this point we were about 29;000 ft. and believing we had a significant fuel leak we felt it most prudent to get the jet pointed at the nearest suitable airfield which was DSM. The Captain coordinated with ATC and [Requested Priority Handling] while I continued to fly. He referenced the Fuel Leak checklist in the QRH and as we were descending through about 24;000 ft. already heading back to DSM; step 3 of the Fuel Leak Checklist directed us to check the cross feed valve. The Captain closed the cross feed valve and informed me of the mistake. We discussed whether or not we had enough fuel to continue on to ZZZ or to return to DSM and agreed that the safer option was to continue to DSM to get more fuel before proceeding to ZZZ. The Captain terminated the emergency with ATC at this point. Back in the chocks we informed DSM personnel and the fire chief of the error and that there was nothing wrong with the aircraft. We coordinated with DSM personnel as well as our Dispatcher to get the flight headed back to ZZZ. The Captain made a phone call to the Duty Officer to inform him of the scenario.In hindsight; here is a list of contributing factors (not intended to be excuses; just an analysis of the occurrence):1. Maintenance leaving switches in abnormal positions than we are used to2. Fact that the fuel quantity was rapidly decreasing on the left side which was the side they had worked on3. Crossflow switch 'shutter' flow bar was faded (we compared to the flow bars in the fuel pump switches and there was a difference; took picture of this after parked.4. Ballast fuel modification to FMS when comparing totalizer to calculated on PROG/2 Cause:1. Maintenance leaving switches in abnormal positions than we are used to2. Fact that the fuel quantity was rapidly decreasing on the left side which was the side they had worked on3. Crossflow switch 'shutter' flow bar was faded (we compared to the flow bars in the fuel pump switches and there was a difference; took picture of this after parked.4. Ballast fuel modification to FMS when comparing totalizer to calculated on PROG/2

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.