Fuel pump

Diesel lift pumps supply the injection pump or common rail high-pressure pump with sufficient filtered diesel fuel. They operate in the low-pressure circuit: insufficient delivery causes starting problems, loss of power and potentially inadequate lubrication or cooling of downstream components. On MAN and Mercedes trucks, buses and vans, both pump pressure and fuel delivery rate must therefore be assessed under load.

Depending on the system layout, mechanically driven gear, vane or plunger pumps are used, alongside electric lift pumps in the tank or fuel line. A hand priming pump is mainly used for filling and bleeding and does not necessarily replace the main lift pump. The position relative to pre-filters, fine filters and water separators also varies by configuration.

Always identify parts using the vehicle's chassis number. Visual comparison or selection based on the model name alone is never sufficient. For MAN D08 and D26 engines or Mercedes OM 906 and OM 471 engines, for example, the engine variant, production period and injection system are decisive. The exact fuel supply configuration must also be checked for MAN TGE and Mercedes Sprinter; an electric pump is not directly interchangeable with a mechanical version.

Check the specified operating pressure in bar, the flow rate in litres per hour at the stated back pressure, the suction capacity and the opening pressure of any pressure regulating or overflow valves. Also compare the direction of rotation, drive fit, flange dimensions and fuel line connections, as well as the supply voltage and control system for electric versions. Internal clearances determine volumetric efficiency; wear on the gears and pump housing increases internal leakage. Seal materials must be suitable for the specified diesel fuel and the permitted biofuel content.

Extended cranking after the vehicle has been standing often indicates fuel draining back through leaking non-return valves or air ingress on the suction side. Loss of power under heavy load is consistent with insufficient fuel delivery, but can also result from blocked filters or a restricted tank pick-up. Water causes corrosion; dirt particles damage pump surfaces. An audible electric pump does not prove that pressure and flow rate are correct.

1. Read out fault codes and available low-pressure readings. Check the fuel level, water separator, filter maintenance, tank ventilation and fuel line routing before condemning the pump.
2. Use a suitable pressure gauge at the specified low-pressure test point during cranking, idling and operation under load. Compare the readings with the vehicle-specific manufacturer's specifications; a universal pressure limit is unreliable.
3. Measure the vacuum upstream of the pump using a vacuum gauge. Excessive suction vacuum indicates a restriction in the tank pick-up, fuel lines or pre-filter. Where possible, measure the pressure differential across the filter.
4. Check for air ingress using a temporary, diesel-resistant transparent test section in accordance with the workshop procedure. A suction line can draw in air without any visible fuel leakage.
5. Measure the flow rate using a suitable fuel flow meter under the specified test conditions. For electric pumps, also check the voltage drop under load and current draw; for mechanical pumps, inspect the drive coupling.

Only replace a worn diesel lift pump after ruling out suction restrictions, air ingress and electrical supply problems. Clean the connection areas before removal, cap open lines and fit new seals as specified. If metal particles or water contamination are present, also check the tank, filters and downstream components; replacing the pump alone leaves the cause of the damage unresolved.

Observe the specified mounting position, drive fit and tightening torques. Fill and bleed the system according to the manufacturer's procedure without allowing the pump to run dry. Do not open common rail high-pressure lines to bleed the system. Then verify leak-tightness, pressure and fuel delivery under load, and check restarting after the vehicle has been standing. Braem can provide technical support with selecting and fitting the correct version.