The sucking jet pump is a part of the fuel tank and pumps fuel from the left-hand tank half into the right-hand half. Since the sucking jet pump delivers the fuel directly into the fuel baffle, it is always filled first, thus ensuring that the electrical fuel pump always has a maximum of fuel available.
The sucking jet pump is driven by the fuel in the return flow line. It is routed via a jet nozzle and accelerated through a narrow cross-section. The fuel flowing at high speed produces a vacuum which pumps the fuel from the left-hand tank half into the right-hand half.
To ensure that the fuel pressure produced by the constricted nozzle in the sucking jet pump cannot rise too high, a pressure relief valve is installed to limit this pressure. Too high a pressure would result in too high a fuel flow rate, thus restricting or cancelling fuel delivery.
A displaced nozzle of the sucking jet pump or a defective pressure relief valve result in the vehicle breaking down due to a fuel deficiency, although fuel is still indicated by the fuel gauge. In both cases fuel is no longer pumped from the left-hand tank half into the right-hand half.