• Fuel is continuously pulled through the Filter Sock by the Lift Pump so that the IST is constantly full.  The Lift Pump's only function is to fill the IST; it does not provide pressurized fuel to the injectors.
  
• Excess fuel spills back into the fuel tank through the Overflow Port
• The Pressure Pump pulls fuel from the IST, through the optional Coarse Filter
• Pressurized fuel is pushed through the Fine Filter to the Fuel Rail, where it is injected into the engine
• After passing through the Regulator, unused fuel returns to the IST
• If the fuel level in the IST drops below approx. 20%, the Fuel Level Switch will open, indicating impending fuel starvation.  This signal can be used to trigger a variety of safety features in the vehicle.

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All ISTs include a Lift Pump, as a minimum.  In the configuration pictured above, the Pressure Pump is located external to the fuel tank.  In general, this is the preferred arrangement:

• Better access for maintenance of the Pressure Pump
• Smaller, less expensive IST

If there is no feasible way to mount the Pressure Pump outside of the fuel tank, it can be packaged inside the tank, as part of the IST.