A fuel surge tank (FST) is designed to prevent fuel starvation to the engine on vehicles with inadequate fuel tank baffling. The FST fuel pump(s) feed the fuel rail(s).
The volume of fuel inside the surge tank acts as a buffer to always keep the FST pump supplied with fuel. This volume is maintained by the main fuel tank's pump as well as the fuel returning from the fuel pressure regulator. Excess fuel in the surge tank is returned back to the vehicle's main fuel tank.