Date: 201810
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying; Other / Unknown
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Ground Event / Encounter Other / Unknown
Detector: Person Maintenance; Person Air Traffic Control; Person Ground Personnel
When detected: Taxi
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On this flight we were scheduled to fly the all nighter from ZZZ to ZZZ1. Everything was going along just fine. We had a normal preflight. On push back we were cleared to start engines and we started both engines with the expectation to have short taxi out. The ground crew had a problem with disconnecting from the aircraft. During the time the ground crew was working on the disconnect we started both engines and actually achieved a five minute warm up. The First Officer; I remember saying; we got our warm up; and we were still being disconnected. The engines achieved a good warm up with no abnormal engine operations before I applied break away thrust.If fuel or any other combustible was pooled in the engine it should have been cleared during the more than 5 minute idle time. Sometime during taxi out the maintenance personnel from [company] noticed that our Number 1; Left Engine torched a 30 - 40 foot flame out the back of the engine. They said this occurred about 7 times as we taxied out and resembled an after burner. They evidently notified the Ground Controller and we were notified before we got to the parallel taxiway. We had no indications; exceedances or alarms in the cockpit. We immediately held our position and an airport operations vehicle; which was nearby; gave us a visual inspection; almost immediately. They said they had witnessed the flames too and that we now looked normal with no further indications of fire or damage.We notified [Maintenance] via a phone patch through Dispatch and we were directed to return to the gate. I made a write up of the event. Maintenance inspected the engine and flap area for damage. We left the flaps down on the return to the gate since the Maintenance people told us they wouldn't be surprised if we had damage to them because of the intensity of the flames. None of our passengers or flight attendants noticed any of the torching. I assume because the cleaners leave the window shades down.Maintenance found nothing unusual with engine; including BITE tests or any damage caused by the torching. They took the plane and did a high powered engine run at the runway. About four hours later; the aircraft was returned to service and we flew it with 3 passengers to be re-crewed. The flight segment continued to ZZZ2. We were out of crew duty and not able to extend.Maintenance said that this was what they called a Candle Flame Anomaly.I was pleased the Maintenance took the time to brief me thoroughly on the procedures they did to clear the write up. I was not going to be an engineering pilot with passengers onboard going to ZZZ2.




Synopsis: B737 Captain reported that during taxi the flight deck crew was informed that flames were seen coming from an engine.