B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 650
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted
Aircraft
In cruise flight; we encountered a potential fuel leak. We were cruising at 36;000 feet. We had been monitoring the fuel against the [planned burn] the entire flight. We had been down very little according to the [planned burn] but we were consistently at the same number. After we crossed some weather; we noticed a large fuel imbalance; approximately 800 pounds (left side low). The captain and I discussed our problem and began to run the Fuel Imbalance Checklist. As we [are] running the checklist; the imbalance became larger than 1;000 pounds and we got the fuel imbalance message. We followed the checklist; including ensuring that the cross feed valve is closed (it was). We monitored the total fuel quantity indication and noted that it was decreasing rapidly. At one point we were down about 2;000 pounds and received a Using RSV Fuel message. The Fuel Imbalance Checklist led us to the Engine Fuel Leak Checklist. We ran the checklist up to the point of engine shut down. We then called [Maintenance Control]; discussed it with them and [requested priority]. We were entering an area of the [mountains] that had few close; suitable alternates. We diverted to [closest airport].
In cruise at 36;000 feet. All systems were normal; we were approaching a cold front and received a reroute by ATC. Approaching the cold front; I had the flight attendants take their jump seats for potential turbulence. As we were passing the front; we noticed the left fuel tank quantity decreasing and eventually had an Imbalance alert. We ran the [Fuel] Imbalance Checklist as well as the Fuel Leak Engine Checklist. Totalizer and calculated fuel were decreasing at an accelerated rate. We received an FMC using reserve fuel message. We had a phone patch call with Maintenance Control. During discussion with Maintenance Control; imbalance increase subsided. At this point arrival fuel was approximately 1;700 pounds less than when the issue first started. Then; the imbalance resumed again slowly with left tank decreasing and imbalance increasing. We were advised Maintenance Control and Dispatch that we would divert before proceeding any further into the inter mountain west. We advised ATC and were cleared direct for an uneventful approach and landing.
B737 flight crew reported diverting after experiencing a fuel system malfunction resulting in a fuel tank imbalance.