Diesel lift pump: what it does and how to tell when it is failing

A diesel lift pump (feed pump, supply pump) draws fuel from the tank and sends it through the filter to the injection pump at low pressure. When it fails, a tractor starts badly, loses power or will not start; a split diaphragm puts diesel in the engine oil. A blocked strainer or a suction pipe drawing air does the same, so test the pump before replacing it.

What does a diesel lift pump do?

A lift pump moves fuel from the tank, through the filters, to the injection pump. A diesel fuel systems training module from Turkey’s Ministry of National Education gives the reason:

“Diesel injection pumps have no suction ability, so fuel has to be sent to them under pressure.”

Ministry of National Education (Turkey), Diesel Engine Fuel Systems module, 2012

The module describes the diaphragm type as the pump a petrol engine uses, with a hand lever added. That lever is the one worked during bleeding, covered in the tractor bleeding article.

How does a lift pump work, and what types are there?

In both tractor manuals we read, the lift pump is a diaphragm pump driven by an eccentric on the engine camshaft. As Massey Ferguson’s MF 65 service manual describes it, the camshaft lifts the pump’s rocker arm, which pulls the diaphragm down against its return spring and creates a vacuum.

Fuel is drawn in through the sediment bowl, the strainer and the inlet valve. On the return stroke the spring pushes the diaphragm up and the fuel out through the outlet valve to the filters.

The module lists four types: piston, diaphragm, gear and vane. Every diesel fuel system has a feed pump, it says; on trucks and construction machines what changes is the type. The piston type is usually mounted on the injection pump, with a separate hand primer at its inlet (a subject of its own).

A common-rail engine may have an electric pump instead. Thermo King says of the one on its common-rail SG 4000 generator set: “This pump is designed to push fuel rather than pull fuel.”

What are the symptoms of a bad lift pump?

A failing lift pump starves the engine of fuel; the symptom ranges from difficult starting to loss of power. The fault-finding chart in Perkins’ workshop manual for the 3.152 series lists “Faulty fuel lift pump” among the possible causes of six faults:

• will not start
• difficult starting
• lack of power
• misfiring
• knocking
• erratic running

All but the knocking row also list a blocked fuel feed pipe, a choked fuel filter and air in the fuel system, so the symptom alone does not point to the pump. Nor does a system that will not bleed: Vintage Tractor Engineer, a specialist site on old tractors, names an inoperative lift pump among the causes.

A diaphragm pump has one symptom of its own:

“The diaphragm can split providing a route for diesel to enter the engine crank case.”

Vintage Tractor Engineer, 14 January 2009

The sign is an oil level that rises for no apparent reason, and thin oil. The site gives two other routes for diesel into the oil: the injection pump shaft seals and leaking injectors.

Inside a diaphragm pump the module lists leaking unions, valves that no longer seal, a holed diaphragm, a weak or broken spring, a worn cam and lever, and a blocked strainer.

How do you test a lift pump?

A lift pump is tested on the engine in two ways: first the flow from its outlet, then its pressure. This is Perkins’ order:

1. Flow. Disconnect the outlet pipe, then turn the engine or work the priming lever. Fuel should spurt “once every two engine revolutions, or every time the hand priming lever is depressed”.
2. If the lever will not go down. Turn the engine one complete revolution; the eccentric on the camshaft has stopped at its highest point.
3. Pressure. Fit a gauge to the outlet and check its connections for leaks. Crank for ten seconds and note the highest reading.

Figures for these two engines only. For any other engine, use its service data.

On the Perkins the test minimum is 75% of the production figure. A second measure: if the pressure falls to half its maximum in less than thirty seconds after cranking stops, the pump needs attention. Before the test, the MF 65 manual says to “make sure the fuel filters are not clogged”.

Is it the lift pump or the fuel line?

If no fuel spurts from the outlet, the fault may lie in the pump or in the line that feeds it. Both manuals measure at the outlet, with the tank, strainer and suction pipe still connected. The manuals do not draw this distinction; it follows from the way the test is set up. Check four places on the suction side:

• Strainer. The Ministry’s tractor module says the feed pump has a strainer for dirt in the fuel, to be cleaned now and then with compressed air.
• Sediment bowl. Perkins wants good joints when the glass bowl is refitted, “as any leakage of air here may cause air locks in the fuel system.”
• Suction pipe. Vintage Tractor Engineer lists blocked pipes and “Split suction pipes allowing air in”. Finding an air leak is covered in the article on air in the fuel line.
• Tank cap. The tractor module says the hole in the cap balances the tank against atmospheric pressure: “This hole must always be kept open and clean.”

The Perkins chart lists a blocked fuel tank vent among the causes of difficult starting, lack of power and erratic running. If the engine picks up when the cap is loosened, look at the vent. The module does not say this; it follows from what the hole is for. What a blocked vent does is in the bleeding article.

An electric pump raises the same question: where one runs but delivers no fuel, Thermo King’s manual points to air leaks, kinked or restricted lines, the bowl gasket and the filter.

What order should you check in?

Follow the fuel: tank first, then the suction side, then the pump.

1. Tank and tap. Fuel in the tank, tap open, cap vent clear?
2. Suction pipe and unions. Look for seepage, a split pipe and loose connections.
3. Sediment bowl and strainer. Drain any water, clean the strainer, seat the gasket properly.
4. Flow. Outlet pipe off, work the lever: fuel at every stroke.
5. Pressure. Gauge on the outlet, against the engine manual’s figure.
6. Engine oil. Level up and oil thin: suspect the diaphragm, the injection pump shaft seals and the injectors.
7. Filters and air. Replace a choked filter, then bleed the system.