Narrative:

Shortly after climbing through 10;000 feet and completing the climb check; we received a fuel imbalance caution. This was confirmed by the left fuel gauge indicating an imbalance of greater than 1;000 pounds and increasing. At that point; we had over 15;000 pounds of fuel in the center tank with all fuel pumps on and fuel crossfeed valve closed. We continued the climb to FL230 where we leveled to analyze the problem. During the climb; the first officer opened up the QRH to the fuel section. Due to the rate of fuel loss from the left tank; we went to the engine fuel leak checklist. In the meantime; I advised the flight attendants not to start a service; as we were troubleshooting a problem. While following the checklist; we called back to the flight attendants to advise them of the problem; and to look at the aft left wing and engine area to see if they could detect a mist coming from either. It was already nighttime and they reported not seeing any mist emanating from the wing or engine. We dispatched with MEL left center tank pump low pressure warning light system.while following the checklist we surmised that an engine fuel leak could not be confirmed; so we called dispatch via arinc to connect with the dispatcher and maintenance control. They were advised of the situation and a consensus was made to divert. I reminded maintenance and dispatch that we would be an overweight landing. By that time; the fuel imbalance grew to approximately 2;000 pounds. By our best estimates; it appeared that we were losing fuel in addition to the normal burn. I advised the flight attendants and passengers of the situation and that we were diverting.the first officer and I coordinated the arrival and approach and furthermore [advised ATC] due to the unknown nature of the problem. Enroute I turned off the center pumps and was able to get the wings in balance by the time we landed. I also noticed at that point that the fuel burn was normal during the balancing; and that the center tank pumps being off seemed to normalize the burn. We determined with the center pumps off the situation did not get aggravated. The landing was smooth and uneventful. After clearing the runway; we stopped the aircraft. The fire department looked over the aircraft and did not see any leaks from the engines or wings. We subsequently taxied to the gate without incident.a logbook entry was made for the fuel imbalance and overweight landing. Subsequently; scheduling called with a reassignment for both pilots to fly one more leg; which we both acknowledged; but advised it would be a while after all parties were debriefed. A conference call was made to the chief pilot on call; maintenance control and dispatch to brief them on the situation. The flight deck crew asked to be removed from the new assignment; which we were.some takeaways from this situation. Both the first officer and I had long days leading up to this assignment although we both felt fit to fly this leg. It made for a fairly long day and a busy divert. Upon further reflection after this event; I believe that the left center fuel pump may have actually had low output; regardless of the MEL; and that it may have not been able to feed center tank fuel to the left engine; if I'm analyzing the fuel schematic correctly. The pump in question did have an electrical draw when switched on. If this was the case; we should have run the fuel imbalance checklist first. Although it would have lead us to the engine fuel leak checklist; had we taken the 'fuel leak not suspected' branch; and if in fact the center pump pressure or plumbing did not allow center fuel to get to the left engine; we might have been able to continue to destination with normal balancing and opening the crossfeed valve. This was not obvious at the time.

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Original NASA ASRS Text

Title: B737-800 flight crew reported that an MEL was not applied correctly causing a fuel imbalance.

Narrative: Shortly after climbing through 10;000 feet and completing the Climb Check; we received a Fuel Imbalance caution. This was confirmed by the left fuel gauge indicating an imbalance of greater than 1;000 pounds and increasing. At that point; we had over 15;000 pounds of fuel in the center tank with all fuel pumps on and fuel crossfeed valve closed. We continued the climb to FL230 where we leveled to analyze the problem. During the climb; the First Officer opened up the QRH to the fuel section. Due to the rate of fuel loss from the left tank; we went to the Engine Fuel Leak Checklist. In the meantime; I advised the Flight Attendants not to start a service; as we were troubleshooting a problem. While following the checklist; we called back to the Flight Attendants to advise them of the problem; and to look at the aft left wing and engine area to see if they could detect a mist coming from either. It was already nighttime and they reported not seeing any mist emanating from the wing or engine. We dispatched with MEL Left Center Tank Pump Low Pressure Warning Light System.While following the checklist we surmised that an engine fuel leak could not be confirmed; so we called Dispatch via ARINC to connect with the Dispatcher and Maintenance Control. They were advised of the situation and a consensus was made to divert. I reminded Maintenance and Dispatch that we would be an overweight landing. By that time; the fuel imbalance grew to approximately 2;000 pounds. By our best estimates; it appeared that we were losing fuel in addition to the normal burn. I advised the Flight Attendants and Passengers of the situation and that we were diverting.The First Officer and I coordinated the arrival and approach and furthermore [advised ATC] due to the unknown nature of the problem. Enroute I turned off the center pumps and was able to get the wings in balance by the time we landed. I also noticed at that point that the fuel burn was normal during the balancing; and that the center tank pumps being off seemed to normalize the burn. We determined with the center pumps off the situation did not get aggravated. The landing was smooth and uneventful. After clearing the runway; we stopped the aircraft. The Fire Department looked over the aircraft and did not see any leaks from the engines or wings. We subsequently taxied to the gate without incident.A logbook entry was made for the fuel imbalance and overweight landing. Subsequently; Scheduling called with a reassignment for both pilots to fly one more leg; which we both acknowledged; but advised it would be a while after all parties were debriefed. A conference call was made to the Chief Pilot on Call; Maintenance Control and Dispatch to brief them on the situation. The Flight Deck Crew asked to be removed from the new assignment; which we were.Some takeaways from this situation. Both the First Officer and I had long days leading up to this assignment although we both felt fit to fly this leg. It made for a fairly long day and a busy divert. Upon further reflection after this event; I believe that the left center fuel pump may have actually had low output; regardless of the MEL; and that it may have not been able to feed center tank fuel to the left engine; if I'm analyzing the fuel schematic correctly. The pump in question did have an electrical draw when switched on. If this was the case; we should have run the Fuel Imbalance Checklist first. Although it would have lead us to the Engine Fuel Leak Checklist; had we taken the 'Fuel Leak Not Suspected' branch; and if in fact the center pump pressure or plumbing did not allow center fuel to get to the left engine; we might have been able to continue to destination with normal balancing and opening the crossfeed valve. This was not obvious at the time.

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.