GM 6.5L V8 – Fuel System – Lift Pump Assembly


An electric lift pump mounts on the inside of the left framerail, using a bracket (see Figure 7-5). The lift pump receives power from a circuit that includes the engine oil pressure switch, a relay and an in-line fuse (see page 8-30 for more information). Wiring from the fuse travels through a bulkhead connector and along the inside of the left framerail to the pump.


The inlet fitting of the lift pump connects to the fuel tank pick-up/sending unit fuel supply fitting, using a pipe/hose assembly with air-tight connections. The outlet fitting of the lift pump connects to the inlet fitting of the fuel filter assembly on the engine, using another pipe/hose assembly.
The electric lift pump has a hollow plunger that slides in a bore located in the center passage between the inlet and outlet ports (see Figure 7-6). 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 (see Figure 7-7). To operate correctly, the injection pump must have fuel with the correct pressure and no air bubbles.
The lift pump is checked as part of fuel supply system diagnosis (see page 7-68). The lift pump should deliver fuel with a minimum volume of 0.24 liter (1/2 pint) in 15 seconds at a pressure of 40 to 60 kPa (5.8 to 8.7 psi).
The air-tightness of the lift pump suction line from the fuel tank is also important for correct operation of the injection pump.


When the pump is at rest, a spring pushes the hollow plunger in the direction of the outlet (see Figure 7-8). Both the inlet and outlet valves are closed, keeping fuel in the supply line from draining back to the fuel tank.
When the pump has electrical power, a solenoid turns ON and pulls the hollow plunger toward the inlet port against spring force (see Figure 7-9, view A). This action causes the inlet valve to open, allowing fuel to enter the pumping chamber.
As the hollow plunger reaches full travel in the direction of the inlet port, the solenoid is turned OFF and spring force pushes the plunger toward the outlet port (see Figure 7-9, view B). This action closes the inlet valve (causing fuel to be drawn from the fuel tank into the pump) and pressurizes the fuel in the pumping chamber (opening the outlet valve to allow fuel to travel to the fuel filter).
As long as the fuel pump has electrical power, the solenoid ON/OFF cycle causes the movement of the hollow plunger and valves necessary to deliver a fuel supply to the filter and injection pump.



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