Date: 201707
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: Shortly after leveling off at FL350 we got a fuel imbalance light. Fuel in the right tank was decreasing way faster than the left tank. We monitored the situation for a few minutes to confirm this. I opened the crossfeed valve (with the FO concurring) and turned off number two fuel pumps as the imbalance was increasing very fast and I could see the control wheel compensating. We read the QRH for a fuel leak. Confirmed there was a leak. Fuel flows were equal. Appeared that right tank was still using more fuel than the left tank. Fuel used was equal. QRH directed us to shut down the engine. We decided not to do that. Main reason for that was that the FAs did not see any fuel leaks anywhere and in my mind I thought if I had misread the situation I did not want to shut down a perfectly good engine and make my problems worse. I didn't want to scare Passengers either. We contacted Dispatcher thru ACARS and then later by radio. We had already started to divert but coordinated it with Company as we were doing that. We landed without incident and did not declare an emergency; but ATC knew what our situation was. We had fire trucks follow us to gate. After sitting for a while and thinking about the situation I was wondering if I had left the cross feed open when I did my preflight. It would have caused a fuel imbalance. When we read the checklist and opened the crossfeed we may have closed it then thinking we had just opened it. We both looked at the switch when I did this but we may have seen what we wanted to see. So when we thought we had opened the crossfeed; and running both engines on the left tank; in reality we may have been running left engine on left tank with boost pumps and the right engine was suction feeding. So it looked like to us; as crossfeeding on left tank and the right tank was still losing fuel like crazy. We diverted right away. The only reason I don't think this is what happened is that I think we would have had a much larger imbalance with the crossfeed open for the entire climb. Our largest imbalance was 1500 pounds; and we actually started to get our imbalance back slightly after we started to crossfeed. Also when we parked the aircraft; we both saw me put the crossfeed to close after I shut down the right engine on taxi.I know we verified the switch movement but I personally did not watch the blue lights activate to verify the valve movement. We also watched the switch being moved but maybe did not really confirm its position. Probably rushed the situation but we thought we had a really bad leak that was uncontrollable. We may have jumped to a conclusion too quickly. Could have probably called Maintenance inflight and gotten some guidance.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew and Dispatcher reported a fuel imbalance at cruise and subsequent diversion.