Fuel Boost Pump / Fuel Transfer Pump: Description and Operation

Figure 3-2: Fuel Lift Pump Construction:

Fuel Lift Pump

The fuel lift pump is located on the firewall near the drivers side of the engine. The electric fuel lift pump has a hollow plunger that slides in a bore located in the center passage between the inlet and outlet ports (Figure 3-2). An inlet valve is mounted on one end of the hollow plunger, and an outlet valve is positioned at the outlet end of the center passage. Both valves are closed by spring force.

The lift pump is designed to move fuel under a low (suction) pressure from the fuel tank and deliver it through the filter to the transfer pump, inside the fuel injection pump. To operate correctly, the injection pump must have fuel at the correct pressure and without air bubbles.

The lift pump is checked as part of the fuel supply system diagnosis. The lift pump should deliver fuel with a minimum volume of one half pint (0.24 liter) in 15 seconds and tee'd in at the injection pump fuel inlet connection running at idle, with a pressure of 5.8 to 8.7 psi (40 to 60 kPa). The lift pump suction line from the fuel tank must be air tight for correct operation of the injection pump. Any air present in the fuel system will cause runability problems and possible diagnostic trouble codes (DTCs) to be set in PCM memory.

Figure 3-3: Fuel Lift Pump Electrical Circuit:

The electrical circuit for the lift pump involves several main components (Figure 3-3). A relay mounted in the exterior fuse box, and the lift pump itself, mounted near the filter on the left side of the engine compartment.

When the driver moves the ignition switch to the "RUN" position, the lift pump relay is energized by the PCM for 5 to 20 sec., depending on PCM time out.

As soon as the PCM receives RPM signals from the EFI pump the relay is energized again. The lift pump remains running as long as the engine runs.