A failing lift pump, also called a transfer or supply pump, causes hard starting, power loss under load, stalling, surging and, on common-rail engines, low rail pressure codes. The lift pump’s job is to deliver fuel to the injection pump at the right pressure and volume, without air. When it can’t, the injection pump starves, and symptoms appear that look like a major injection failure. On some systems, a weak lift pump also causes that failure.
Testing the lift pump is quick and should come before any injection pump or injector replacement.
What the lift pump does
The lift pump pulls fuel from the tank, pushes it through the filters and delivers it to the injection pump. Depending on the engine, it may be:
- An electric pump in the tank
- An electric pump on the frame or engine
- A mechanical pump driven by the engine
- A gear pump built into the high-pressure pump, which pulls fuel from the tank
Many injection pumps rely on incoming fuel for lubrication and cooling. The Bosch VP44 is the classic example: low supply pressure is one of the most common causes of VP44 failure. See Cummins VP44 pump failure.
Symptoms of a weak lift pump
- Long cranking or no start, especially after sitting
- Power loss under heavy load, when fuel demand is highest
- Stalling, sometimes intermittently
- Surging or uneven running
- Low rail pressure codes such as P0087 on common-rail engines
- A pump that’s louder or quieter than usual, or silent at key-on on electric systems
- Problems that get worse with a low fuel level or high fuel temperature
How to test it
Pressure
Connect a gauge at the specified test point and compare readings at key-on, idle and, if possible, under load with the engine maker’s specification. A pump that makes pressure at idle but falls off under load is weak.
Volume
Some engine makers specify a volume test: how much fuel the pump delivers in a set time. A pump can make pressure but not enough flow.
Suction-side restriction
Measure restriction on the suction side if the procedure calls for it. A clogged tank screen, collapsed hose or plugged primary filter makes a good pump look bad.
Air
Use a clear section of line at the injection pump inlet to look for bubbles. Air in the supply has its own causes; see air in the diesel fuel system.
Electrical
On electric pumps, check voltage at the pump while it runs and look for voltage drop in the wiring, relays and grounds. A pump starved of voltage can’t deliver.
What a weak lift pump can damage
- Injection pumps that depend on supply fuel for lubrication and cooling
- High-pressure common-rail pumps, which can be damaged by running starved or on aerated fuel
- Injectors, indirectly, through poor rail pressure and erratic operation
Replacement and upgrades
Replace a failed lift pump with one that meets the engine maker’s specification for pressure and flow. Aftermarket supply systems are popular on some trucks; if you choose one, make sure it delivers pressure within the range the injection system expects, since too much can be a problem as well as too little.
Lift pump problem or injection pump problem?
| Clue | Leans lift pump/supply | Leans injection pump |
|---|---|---|
| Supply pressure low or falls under load | ✓ | |
| Problem worse with low fuel level | ✓ | |
| Electric pump silent at key-on | ✓ | |
| Supply pressure normal, rail or injection pressure low | ✓ | |
| Metal in the fuel filter | ✓ | |
| Hard starting hot with good supply pressure | ✓ |
Measure supply pressure before deciding. It’s one of the simplest tests in diesel diagnosis and one of the most frequently skipped.
Further technical reference: Bosch Mobility: common-rail system overview. Use the exact engine/injector/pump service information for applicable procedures and limits.
If the injection pump suffered
If the lift pump has been failing for a while, the injection pump may have been damaged. Diesel Control Technicians tests injection pumps and injectors to show whether low supply pressure took a toll. Send DCT your supply pressure readings along with the components, or schedule an on-vehicle diagnosis by appointment in Kansas.
Discuss injector testing: Request a mail-in component review with the complete injector number, original cylinder, symptoms and any existing measurements. Confirm the supported test and repair path before removal or shipment.