Narrative:

Pushback and engine start were normal. Ground operations did not report any issues when I cleared them off headset. [We] taxied to runway 22L for departure. After receiving and complying with a lineup and wait clearance; tower advised us that a city operations vehicle had observed us trailing some type of fluid during taxi. They did not know the source or type of fluid. All cockpit indications of fluid quantity and pressure were normal. We coordinated with tower to have the city operations vehicle drive around the aircraft to try to determine the source of the leak while we remained in position on the runway. After driving around the aircraft; the city operations employee pointed at the # 2 engine; and through hand signals; indicated that there was a significant leak from that engine. I told the first officer to ask for a clearance to return to the gate and I secured the # 2 engine. We returned to the gate on one engine; and as we joined the parallel taxiway to return; we noticed a significant trail of fluid from our taxi out that I would estimate was around one foot wide. As we approached the gate; I noticed a puddle of fluid where we had held during engine start and tow bar disconnect. The puddle was approximately 50 feet in diameter. We deplaned all passengers for an engine run that was coordinated with maintenance. After the engine run was completed; the aircraft was taken out of service for a significant fuel leak at idle.

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Original NASA ASRS Text

Title: After taxiing into position for takeoff; B737-700 flight crew was informed by a ground vehicle of fluid leaking from the Number Two engine. Upon return to gate; a significant fuel leak is discovered.

Narrative: Pushback and engine start were normal. Ground Operations did not report any issues when I cleared them off headset. [We] taxied to Runway 22L for departure. After receiving and complying with a lineup and wait clearance; Tower advised us that a city operations vehicle had observed us trailing some type of fluid during taxi. They did not know the source or type of fluid. All cockpit indications of fluid quantity and pressure were normal. We coordinated with Tower to have the city operations vehicle drive around the aircraft to try to determine the source of the leak while we remained in position on the runway. After driving around the aircraft; the city operations employee pointed at the # 2 engine; and through hand signals; indicated that there was a significant leak from that engine. I told the First Officer to ask for a clearance to return to the gate and I secured the # 2 engine. We returned to the gate on one engine; and as we joined the parallel taxiway to return; we noticed a significant trail of fluid from our taxi out that I would estimate was around one foot wide. As we approached the gate; I noticed a puddle of fluid where we had held during engine start and tow bar disconnect. The puddle was approximately 50 feet in diameter. We deplaned all passengers for an engine run that was coordinated with Maintenance. After the engine run was completed; the aircraft was taken out of service for a significant fuel leak at idle.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.