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In fact the fuel is getting "sucked" into the engines by the engines fuel pumps through a dedicated set of bypass lines - however, this fuel is getting sucked out of the wingtanks, only. So if you have fuel in the center tank, and BOTH of the center tanks pumps fail, you could be in a difficult situation: The center fuel becomes unusable, and you may be getting into trouble with your trim - using the fuel from the wings only will drive your CG forward, possibly out of limit. So even if your fuel remaining in the wingtanks is high enough to reach a comfortable alternate or even your destination, CG considerations may require you to land much sooner than that. Jan

Hallo Jan, danke für die Klarstellung!

the aircraft has a scavenge jet pump that drains the little remaining fuel on the center tanks to main tank 1 so all fuel in the center tank is used. On the classic series 737-300, the scavenge jet pump activates manually by the pilot when you turn off the center tank fuel pump, and it works for 20 minutes, transfer rate is very low. On the 737-NG the process is automatic, when the center tank is low and the main tanks reaches half their capacity, the scavenge pump activates and drains remaining fuel to main tank 1, process runs for the rest of the flight.