Narrative:

We were level at FL360 on a return leg to lax. A little over 1 hour into the flight; the captain opened the fuel xfeed valve and turned off a center tank fuel boost pump in accordance with our procedure with less than 2000 pounds remaining in the center tank. Several mins after that; we got a fuel imbalance warning light. The imbalance was around 1100 pounds. The captain referenced the QRH and started the rebalancing procedure. That seemed to be working somewhat; however; the left side tank (which was the low side) was still feeding when it should not have been. This puzzled us and I started trying to call our company maintenance on VHF with no luck. The imbalance was improving though; and had decreased to around 900 pounds. When the captain started the rebalancing procedure; the center tank which had 1300 pounds remaining stopped feeding as expected. We decided to turn those center pumps back on to burn that fuel which caused the wing tanks to stop feeding as expected. So we burned the remaining center fuel and then went back to trying to rebalance the wing tanks which still had about a 900 pound imbalance. I continued to try and call our company on both VHF and HF with no luck. Now with the rebalancing in effect; the imbalance started getting worse with the left side feeding at least twice as fast as the right side which was the high side. We also discovered that the xfeed light on the fuel panel was inoperative even after we changed the bulbs. We started to think we either had a possible leak or trapped fuel in the right tank. We started to think about possible divert fields and realized the field at ZZZ1 was about 40 mi away while ZZZ2 in the united states was about 250 mi away. I sent both our dispatcher and maintenance an ACARS message about our problem and they both acknowledged with generic responses. The imbalance had now gotten back to about 1100 pounds and was continuing to worsen. We then decided to divert into ZZZ1 because we weren't sure we had enough usable fuel to get us to ZZZ2 or how large the imbalance would be when we got there. During the descent either maintenance or the dispatcher sent us an ACARS with a possible solution to the problem; however; we were well into the divert; did not take time to investigate further. Once on the ground at ZZZ1; the captain talked with maintenance on the phone and they directed us to check the fuel xfeed circuit breaker behind the first officer's seat. We found the breaker was out and contract maintenance was able to come out and reset the breaker and return the aircraft to service.

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Original NASA ASRS Text

Title: B737-800 FLT CREW EXPERIENCED A FUEL IMBALANCE ENROUTE. UNABLE TO BALANCE THE FUEL; THE FLT CREW DECLARED AN EMER AND DIVERTED.

Narrative: WE WERE LEVEL AT FL360 ON A RETURN LEG TO LAX. A LITTLE OVER 1 HR INTO THE FLT; THE CAPT OPENED THE FUEL XFEED VALVE AND TURNED OFF A CTR TANK FUEL BOOST PUMP IN ACCORDANCE WITH OUR PROC WITH LESS THAN 2000 LBS REMAINING IN THE CTR TANK. SEVERAL MINS AFTER THAT; WE GOT A FUEL IMBALANCE WARNING LIGHT. THE IMBALANCE WAS AROUND 1100 LBS. THE CAPT REFED THE QRH AND STARTED THE REBALANCING PROC. THAT SEEMED TO BE WORKING SOMEWHAT; HOWEVER; THE L SIDE TANK (WHICH WAS THE LOW SIDE) WAS STILL FEEDING WHEN IT SHOULD NOT HAVE BEEN. THIS PUZZLED US AND I STARTED TRYING TO CALL OUR COMPANY MAINT ON VHF WITH NO LUCK. THE IMBALANCE WAS IMPROVING THOUGH; AND HAD DECREASED TO AROUND 900 LBS. WHEN THE CAPT STARTED THE REBALANCING PROC; THE CTR TANK WHICH HAD 1300 LBS REMAINING STOPPED FEEDING AS EXPECTED. WE DECIDED TO TURN THOSE CTR PUMPS BACK ON TO BURN THAT FUEL WHICH CAUSED THE WING TANKS TO STOP FEEDING AS EXPECTED. SO WE BURNED THE REMAINING CTR FUEL AND THEN WENT BACK TO TRYING TO REBALANCE THE WING TANKS WHICH STILL HAD ABOUT A 900 LB IMBALANCE. I CONTINUED TO TRY AND CALL OUR COMPANY ON BOTH VHF AND HF WITH NO LUCK. NOW WITH THE REBALANCING IN EFFECT; THE IMBALANCE STARTED GETTING WORSE WITH THE L SIDE FEEDING AT LEAST TWICE AS FAST AS THE R SIDE WHICH WAS THE HIGH SIDE. WE ALSO DISCOVERED THAT THE XFEED LIGHT ON THE FUEL PANEL WAS INOP EVEN AFTER WE CHANGED THE BULBS. WE STARTED TO THINK WE EITHER HAD A POSSIBLE LEAK OR TRAPPED FUEL IN THE R TANK. WE STARTED TO THINK ABOUT POSSIBLE DIVERT FIELDS AND REALIZED THE FIELD AT ZZZ1 WAS ABOUT 40 MI AWAY WHILE ZZZ2 IN THE UNITED STATES WAS ABOUT 250 MI AWAY. I SENT BOTH OUR DISPATCHER AND MAINT AN ACARS MESSAGE ABOUT OUR PROB AND THEY BOTH ACKNOWLEDGED WITH GENERIC RESPONSES. THE IMBALANCE HAD NOW GOTTEN BACK TO ABOUT 1100 LBS AND WAS CONTINUING TO WORSEN. WE THEN DECIDED TO DIVERT INTO ZZZ1 BECAUSE WE WEREN'T SURE WE HAD ENOUGH USABLE FUEL TO GET US TO ZZZ2 OR HOW LARGE THE IMBALANCE WOULD BE WHEN WE GOT THERE. DURING THE DSCNT EITHER MAINT OR THE DISPATCHER SENT US AN ACARS WITH A POSSIBLE SOLUTION TO THE PROB; HOWEVER; WE WERE WELL INTO THE DIVERT; DID NOT TAKE TIME TO INVESTIGATE FURTHER. ONCE ON THE GND AT ZZZ1; THE CAPT TALKED WITH MAINT ON THE PHONE AND THEY DIRECTED US TO CHK THE FUEL XFEED CIRCUIT BREAKER BEHIND THE FO'S SEAT. WE FOUND THE BREAKER WAS OUT AND CONTRACT MAINT WAS ABLE TO COME OUT AND RESET THE BREAKER AND RETURN THE ACFT TO SVC.

Data retrieved from NASA's ASRS site as of May 2009 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.