Date: 202107
Local time of day: 1201-1800
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: Ground ZZZ; Tower ZZZ
Aircraft component: Air Conditioning Distribution System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During preflight in 44C temperature day cargo smoke detectors were acting erratic and not testing in various configurations of the panel. Maintenance thought it might be heat related and ducted conditioned air into forward bin. The forward bin abnormalities stopped ;either by cooling the bin or exercising the detectors switches. The aft bin wasn't cooled and the mix meld the aft cargo smoke detector system. Plane is signed off and we push back. With 2 engines running and the APU on for a bleeds off takeoff; we started to taxi out to runway X Left full length. Bleeds were configured eng1 off; APU on; eng2 on. Isolation valve closed and packs  on. About 4 minutes after engine start near [intersection] the aft FA calls the flight deck. FO answers call and I stop the plane. I turn on service interphone switch to hear an FA saying electrical/ burning plastic smell in the aft galley. I grab phone and ask where she is smelling it from; initially she doesn't know and says it is faint but getting stronger.  I asked her to pull all of the circuit breakers in the galley. I decided to return to the gate and we advised ATC. During taxi back we started smoke;fire;fumes checklist. The utilities/ife were switched off transfer bus off recirculating fans off WiFi off. The FA calls back and says it is coming out of the gasper vents in the galley and it was getting stronger after I turned off the recirculating fans. We were waiting for parking crew at this point and in 44c temps I was trying to keep as much cool air as possible. Eng bleeds were both turned off. APU bleed on ; left pack high ; iso closed. We could not smell anything in the front; so I figured it wasn't from the left pack or APU. At the gate; ground air was hooked up and the second the jetway was up and door opened I instructed via the pa for all passengers to grab their belongings and exit the plane. After the passengers deplaned; I went to the back and could smell the residual coming from the gasper. It was a melting plastic smell. It faded; to my dismay; before the maintenance technician got to us. They turned on all systems; and even performed an engine run to find the cause. As they were unable to duplicate the smell; they signed it off as such. By this time the fuel was up to 54C and we were unable to operate that flight due to time. We were put on another flight and a separate crew operated on a different plane.




Synopsis: B737 Captain reported a fume event during taxi resulting in ground gate return.