B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Departure Airport
Aircraft
We took off with fuel balanced and approximately 8;300 pounds per tank. During climb out we were focused on weather deviation and communicating with the Flight Attendants to keep them seated. At level off at FL360 we noticed the yellow imbalance annunciation on the fuel panel; and noted the readings to be 5;600 pounds in the left tank (and dropping rapidly) and 7;100 pounds in the right tank (burning normally). We initiated an immediate turn around back to [departure airport] with Center; unsure if there was a leak or some other malfunction responsible for the huge imbalance. They also gave us a descent.We began running the Fuel Leak [Checklist]. When we reached the point of the checklist requiring confirming an engine fuel leak we noted the fuel level was no longer dropping as rapidly. We asked the Flight Attendants to inspect for signs of a leak and they reported they didn't see anything out of the ordinary. We elected to leave both engines running; as at that point we were being vectored on to final approach. We landed uneventfully; but noted as we cleared the runway it looked like the left side was dropping rapidly again. We shut down the #1 engine and asked the Airfield Ops vehicles on scene to inspect our engine for signs of a leak. They reported no evidence of a leak. We taxied back to the gate. Our Passengers deplaned normally; and Maintenance took the aircraft from us. They also said there were no immediate signs of a leak; but would have to troubleshoot to find the cause of the imbalance.
Boeing 737 First Officer reported fuel system issue and returned to the departure airport