A lift pump is the low-pressure fuel pump that draws diesel from the tank, pushes it through the filters and supplies it to the high-pressure injection pump. On common-rail diesels such as the 5.9 and 6.7 Cummins, the high-pressure pump depends on a steady supply from the lift pump; a weak lift pump can starve it, causing hard starting and lost power, and over time it can shorten the life of the expensive high-pressure pump.
What a lift pump does
- Draws fuel from the tank so the high-pressure pump never has to pull it.
- Pushes fuel through the fuel filter and water separator.
- Maintains a supply pressure the high-pressure pump needs to run cool and lubricated.
Lift pump failure symptoms
- Hard starting or long cranking
- Loss of power under load, especially when towing
- Surging or stalling
- Low fuel pressure codes; on common-rail trucks, low rail pressure such as P0087; see CP3 pump
- A pump that no longer hums when the key is turned on
How to check supply pressure
A fuel pressure gauge at the filter head or test port shows the supply pressure at idle and under load; compare it with the specification for your engine. Check for clogged filters, a restricted pickup in the tank and wiring problems before replacing the pump, because they produce the same symptoms.
Why it matters on the Cummins
The 24-valve 5.9 (1998.5-2002) used the VP44 electronic injection pump, which is widely reported to suffer when the lift pump is weak. Common-rail engines use a CP3 or later pump that also relies on a good supply. Replacing a failed injection pump without fixing a weak lift pump risks a repeat failure. Many owners fit a gauge so they can watch supply pressure; see Cummins generations for which pump your truck has.
Aftermarket lift pumps
Many diesel owners fit aftermarket lift pumps with higher flow, extra filtration and air or water separation, especially on modified or towing trucks. Choose one designed for your engine and injection system, keep supply pressure within what the system expects, and route the wiring and lines as the instructions describe.
Keeping the fuel system healthy
- Change fuel filters on schedule and drain the water separator.
- Avoid running the tank very low.
- Fix air leaks on the suction side, which cause rough running and starting problems.
Complete engine packages: the D&J Ready Run engines on our Cummins engines page include the high-pressure fuel system. See also 6.7 Cummins specs.
More guides: fuel filter and water separator · CP3 vs CP4 · diesel losing power · diesel won’t start · diesel fuel gelling · diesel knocking · how to bleed a diesel fuel system · water in diesel fuel · how long a fuel pump lasts.
Trouble codes: P0087 Fuel Rail Pressure Too Low · P0191 Fuel Rail Pressure Sensor Range · P0089 Fuel Pressure Regulator Performance · all OBD2 codes.
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