Fuel Leak B777
I know this is an old thread but Boeing still haven't changed the checklist.
In my humble opinion the only way to determine if it is an engine leak or a tank leak is to pull the fire handle after shutting down the engine. This closes the pylon valve. If the leak continues then it is a tank leak. If it stops it is an engine leak.
It is my understanding that certification requirements define an engine leak as being between the pylon valve and the engine.
Clearly if it is a tank leak the engine may as well be restarted. Boeings checklist is very poor in this regard. It will save the day inall cases but can result in unnecessary single engine operations.
Some will argue you dont need to pull the fire handle and that once the engine is shut down continued leakage is a tank leak. This is not correct as there is still stuff between the fuel cuttoff valve and the pylon valve that could be leaking.