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GM/Detroit 6.5L – Fuel System – LIFT 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 o-rings. 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 (figure 4-2). 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 0.24 liter (1/2 pint) in 15 seconds and tee-d in running pressure of 40 to 60 kPa (5.8 to 8.7 psi) for 1988-1993 pumps or, on 1994 and later pumps, a minimum of 3 psi with the line from the filterto the injection pump open to an approved canister and the engine cranking (i.e. not running). The lift pump suction line from the fuel tank must be air tight for correct operation of the injection pump.

The electrical circuit for the lift pump involves several main components (figure 4-3):
• The oil pressure switch/sending unit, threaded into a lubrication system passage at the rear of the
cylinder case
• A relay mounted near the junction block on the vehicle cowl
• An in-line fuse mounted near the battery junction block on the passenger side of the cowl
• The lift pump itself, mounted under the vehicle on the inside of the left frame rail
• Terminal G of the Data Link Connector (DLC)

When the driver moves the ignition switch to the CRANK position, the lift pump circuit is completed through the relay contacts. During this time, oil pressure is building to the point of closing the contacts of the pressure switch/sending unit.
A minimum of 28 kPa (4 psi) is required to close the switch contacts.

When the ignition switch is returned to the RUN position, the oil pressure of the running engine maintains electrical power to the lift pump. If engine oil pressure drops below 28 kPa (4 psi), the engine may run poorly or stall when the lift pump circuit opens.


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