The center tank pumps were switched off as part of the investigation process, so that each engine is fed via its associated inner tank only.
Since it is determined that the leak is not from either inner tank nor engine/pylon (both inner deplete in a similar rate), what is left is either APU line or center tank. Supposing center tank leak I really see no reason why not selecting center tank pumps on (mode sel auto) in order to use at least some amount of the center tank. Waiting for full inner tanks to go below 3t, might lead to loss of center tank fuel, part of which could have been used. Auto mode would still control center pumps on-off to allow IDG cooling fuel return to the wings.
@Tyropicard
Actually even with full center tank feeding engines when inners are at 3t, I am not sure if fuel return is able to top up the wings again. I mean fuel has to be used also by the engines. On the other hand if center tank pumps were selected on after the investigation, inner tanks would remain full or almost full until center tank went empty (fuel either used or lost).
@dustypfd
Good question. I don't know the answer. Any help appreciated
