Narrative:

To my knowledge; there was no wrongdoing by the crew. This report is for information because the day followed an odd chain of circumstances regarding the fuel system on this aircraft. My day trip pairing was to fly to ZZZ; then flight ZZZ1 to return in the same airplane. Release 1 for flight ZZZ2 initially showed a fuel load of about 16.4; but was updated to release 2 with an added alternate and new fuel load of about 17.2. At the gate the fuel gauges initially read 16.4 but showed an imbalance with the left main tank showing 1000 pounds less than the right main tank. We called maintenance out to refasten an unrelated engine cowling latch; and since they were coming out anyway I asked if they would check with the fueler to assure everything was alright with the fuel system since the imbalance is not the way we normally fuel this airplane. The mechanic came on the flight deck to say the fueler was not out there and if we didn't have enough fuel to call ops since this was an ops/fuel issue and not a maintenance issue. Something still didn't feel quite right to me but I couldn't argue. We received the new fuel number we needed per release 2; the fuel was put into the left main tank; we no longer had an imbalance; everything seemed fine; we forgot about the whole thing and proceeded on our way. Airborne I turned on the center tank pumps to drain out the minimal amount of 350 pounds that was there (3 minutes of fuel). Both center fuel pump low pressure lights illuminated so I turned them back off which is procedure when draining the center tank. Usually it'll burn that amount of fuel but I honestly didn't think much of it; and figured that small amount would suction feed into the other tanks. We landed uneventfully in ZZZ. For the return flight ZZZ1 we were fueled with slightly more than the previous leg; total about 17.9 which put 850 pounds in the center tank. Again; as with the previous leg; there was not enough in the center tank to allow burning it down while on the ground; limitations prevent burning center tank fuel on the ground when it is less than 1000 pounds. Airborne; I turned on the center pumps to burn the 850 pounds. Both low pressure lights illuminated again and the fuel would not burn. This time it definitely got my attention as this fuel most assuredly should have burned. I tried resetting the pumps and looked for popped cbs (circuit breakers); but could find no explanation. We had plenty of fuel to make it to ZZZ3 with no issues; no limitations were exceeded; there was nothing preventing safe operation of this flight; but we were compromised in that we had 850 pounds of unusable fuel onboard. Circumstantially; it worked out okay for us; but had this been any other flight a diversion and/or limitation exceedance would have been likely. I wrote it up and was met at the gate by the same mechanic who I had spoken to earlier that morning regarding the main tank imbalance. I do not know if the morning's fueling abnormality was related as I do not know precisely how fuelers do their job; whether valves were repositioned for the imbalance and not reset; or if this was coincidence. I left the airplane in the hands of the mechanic and went on my way.

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Original NASA ASRS Text

Title: B737 Captain reported fuel system abnormalities.

Narrative: To my knowledge; there was no wrongdoing by the crew. This report is for information because the day followed an odd chain of circumstances regarding the fuel system on this aircraft. My day trip pairing was to fly to ZZZ; then flight ZZZ1 to return in the same airplane. Release 1 for flight ZZZ2 initially showed a fuel load of about 16.4; but was updated to Release 2 with an added alternate and new fuel load of about 17.2. At the gate the fuel gauges initially read 16.4 but showed an imbalance with the left main tank showing 1000 pounds less than the right main tank. We called Maintenance out to refasten an unrelated engine cowling latch; and since they were coming out anyway I asked if they would check with the Fueler to assure everything was alright with the fuel system since the imbalance is not the way we normally fuel this airplane. The Mechanic came on the flight deck to say the Fueler was not out there and if we didn't have enough fuel to call Ops since this was an Ops/fuel issue and not a maintenance issue. Something still didn't feel quite right to me but I couldn't argue. We received the new fuel number we needed per Release 2; the fuel was put into the left main tank; we no longer had an imbalance; everything seemed fine; we forgot about the whole thing and proceeded on our way. Airborne I turned on the center tank pumps to drain out the minimal amount of 350 pounds that was there (3 minutes of fuel). Both center fuel pump low pressure lights illuminated so I turned them back off which is procedure when draining the center tank. Usually it'll burn that amount of fuel but I honestly didn't think much of it; and figured that small amount would suction feed into the other tanks. We landed uneventfully in ZZZ. For the return flight ZZZ1 we were fueled with slightly more than the previous leg; total about 17.9 which put 850 pounds in the center tank. Again; as with the previous leg; there was not enough in the center tank to allow burning it down while on the ground; limitations prevent burning center tank fuel on the ground when it is less than 1000 pounds. Airborne; I turned on the center pumps to burn the 850 pounds. Both low pressure lights illuminated again and the fuel would not burn. This time it definitely got my attention as this fuel most assuredly should have burned. I tried resetting the pumps and looked for popped CBs (Circuit Breakers); but could find no explanation. We had plenty of fuel to make it to ZZZ3 with no issues; no limitations were exceeded; there was nothing preventing safe operation of this flight; but we were compromised in that we had 850 pounds of unusable fuel onboard. Circumstantially; it worked out okay for us; but had this been any other flight a diversion and/or limitation exceedance would have been likely. I wrote it up and was met at the gate by the same mechanic who I had spoken to earlier that morning regarding the main tank imbalance. I do not know if the morning's fueling abnormality was related as I do not know precisely how fuelers do their job; whether valves were repositioned for the imbalance and not reset; or if this was coincidence. I left the airplane in the hands of the mechanic and went on my way.

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.