A diesel injection pump sends diesel to the injectors at high pressure, and it is usually what people mean by a truck "fuel pump". A failing pump shows up as hard starting, power loss, rough idle, black smoke, external diesel leaks or diesel in the engine oil. Filters, air in the fuel or the lift pump can cause the same symptoms, so diagnosis starts on the supply side.
In short:
- For hard starting and power loss, check the tank, filters, air intake and supply pressure first; if that side is clean, move on to the injection pump.
- An external diesel leak most often comes from fitting washers, the overflow valve and line connections; the pump body is a less common source.
- The overflow valve and the drive coupling are replaced from outside the pump; a fault inside the pump needs measurement on a test bench.
- If diesel has got into the engine oil, do not run the engine until the cause is fixed and the oil is changed.
What is a diesel injection pump on a truck, and what types are there?
A diesel injection pump raises diesel to high pressure and sends the right quantity to the injectors at the right time for the engine's needs. A truck fuel system generally has three separate pumps: the lift pump (feed pump), which draws diesel from the tank and pushes it through the filters at low pressure toward the injection side; the hand primer pump, pressed by hand when bleeding air; and the injection (high-pressure) pump, which takes the pressure up to hundreds of bar and beyond.
On a truck the lift pump is usually an engine-driven mechanical pump of the piston or gear type.
Four system types are found on heavy-duty vehicles:
- Inline pump: Each cylinder has its own pumping element; common on older trucks with mechanical control. Most of these pumps are lubricated from the engine's oil circuit; on some engines the drive is connected through a coupling, and a governor and a timing advance device may be fitted on the pump.
- Distributor (rotary) pump: A single high-pressure unit distributes diesel to all the cylinders in turn; the lift pump and governor are inside the housing. It is used more on light and medium-duty vehicles.
- Unit pump and unit injector: Each cylinder's pump is driven from the engine camshaft; a unit pump connects to the injector through a short line, while in a unit injector both are in one housing. There is no central injection pump, and the lift pump supplies the low-pressure side.
- Common rail: The high-pressure pump fills a shared high-pressure line (the rail), and injectors controlled by the engine control unit do the injecting.
The system type is determined from the engine code and the service documentation; what applies to the coupling, injection timing and the overflow valve changes accordingly.
What are the symptoms of a bad diesel injection pump?
Most diesel injection pump failure symptoms do not point to the pump by themselves; the same picture can come from the supply, air and electronic sides too.
| Symptom | Likely cause | Check | What to do |
|---|---|---|---|
| Hard or late starting, not starting after sitting | Air entering on the suction side, overflow valve stuck open, wear in the pumping element or governor, fuel shutoff solenoid (if fitted) | Bubbles in a clear hose, supply pressure; whether the problem returns after bleeding | Fix the air entry, check the valve; if supply is normal, have the pump tested |
| Loss of power, weak on grades | Low supply pressure from a clogged filter, overflow valve, injection timing drift, governor or control rack fault | Supply pressure, timing marks, fault code | Replace the filter or valve, adjust timing, test the pump |
| Rough idle, misfire | Air in the diesel, an element of an inline pump not delivering equally, an injector | Cylinder cut-out test at a shop | Rule out an injector fault; have the pump calibrated if needed |
| Black smoke, knock | Timing error (loose coupling bolt, cracked disc), fuel quantity setting, injector | Timing marks and the coupling | Replace the coupling, adjust timing |
| Diesel wetness around the pump | Fitting washers, overflow valve fitting, line connections, pump seals | Clean and trace the leak (see below) | Replace washers, seals or lines |
| Oil level rising, oil thin and smelling of diesel | Inline or unit pump: element or lift pump leak; unit injector and common rail: injectors, sealing rings or the high-pressure pump | Oil level, smell, oil analysis | Go to a shop without running the engine |
| Engine suddenly stops while driving and won't restart | Broken coupling, fuel shutoff solenoid, fuel supply cut | Whether the pump shaft turns, whether the solenoid gets power; tank and filter | Repair the part concerned |
Power loss and black smoke do not come from the fuel side alone; a charge air leak or a turbo and turbo hose failure gives the same symptoms. On engines with unit pumps, unit injectors, common rail or an electronically controlled pump, the first step is to read the fault code; the pump is not replaced before the code is read. Misfire or smoke on a single cylinder points more to an injector.
How do you tell it's the pump, and where should the check start?
Before moving to the injection pump, rule out the supply side; insufficient or aerated diesel gives the same symptoms as a pump fault, and even a small hole in the suction line can let air into the system. The check runs in this order:
- 1. Tank and suction: fuel level, suction strainer, dented or cracked hose.
- 2. Filters and water separator: clogging and water. Replacement and bleeding steps are covered in the diesel fuel filter and bleeding guide.
- 3. Air entry: filter cover seal, fittings and clamps. If bubbles appear in a clear hose temporarily fitted in the suction line, suspect an air leak.
- 4. Lift pump and hand primer: a lift pump (feed pump) and hand primer pump with a leaking check valve or a piston that seeps can send aerated diesel to the injection side.
- 5. Pressure and return: supply pressure is measured with a gauge, and the value is taken from the manufacturer's service documentation; whether there is flow from the overflow valve to the return line is also checked.
If the supply pressure is normal and the diesel is free of air, the next suspects are the pump, the injection timing and the electronics. The passenger-car habit of listening for the pump and checking the relay and fuse does not apply to an engine-driven lift pump; the service documentation shows whether the vehicle has an electric auxiliary pump.
Why does an injection pump leak diesel?
An external leak at the pump usually comes from sealing points; the housing itself is a less common source. Common leak points are:
- Copper or aluminum washers on fittings and banjo connections; a removed washer is normally replaced with a new one.
- The overflow valve and the banjo fitting it connects to.
- High-pressure lines and their nipples: vibration, missing or broken clamps, wrong tightening.
- Pump housing and cover gaskets, shaft seal.
- The lift pump and hand primer pump mounted on the pump.
- Return line hoses and connections.
Safety: Do not feel for a high-pressure leak by hand with the engine running, and do not loosen high-pressure line fittings to bleed air. A fine jet of diesel can penetrate the skin; because a small-looking wound can cause serious tissue damage, get medical help immediately after such an injury.
Diesel dripping onto a hot exhaust or turbo can start a fire, so the engine is not run and the truck is not driven until the fuel leak is fixed. On high-pressure systems such as common rail, after the engine stops observe the manufacturer's waiting time before starting any removal, and wear safety glasses and gloves.
To find the leak, clean and dry the engine, then trace it from top to bottom; diesel dripping from above can make a fitting below look guilty. If the leak shows only with the engine running, do the check briefly at idle, and hold a piece of cardboard or paper near the suspect area instead of your hand to show dripping or spraying diesel. Do not try to stop a leaking high-pressure line by overtightening it; some manufacturers require the line to be replaced in this case.
A leak is not always external: on inline and unit pumps lubricated with engine oil, diesel can also pass into the engine oil (details in the symptom table above).
What do the overflow valve and the drive coupling do, and what happens when they fail?
The overflow valve is a spring-loaded valve that holds the supply pressure on the injection side at a set value and sends excess diesel and air to the return line. The spring inside sets its opening pressure. On inline pumps it is usually in the pump housing; on some unit-injector engines an overflow valve also regulates the supply pressure to the injectors, and its location is found in the service documentation.
If the valve stays open or its spring weakens, supply pressure drops; hard starting and power loss under load can result. If it sticks closed or clogs, the return flow stops, air cannot be purged from the system and pressure rises; the valve fitting can also leak. At replacement, an overflow valve with the same opening pressure is chosen; because two valves that look identical on the outside can have different opening pressures, it is not fitted unless the OEM number matches.
The coupling is the flexible connection between the injection pump and the engine drive; on adjustable types the injection timing is set through the coupling. Its design varies with the engine and pump; on disc types the flexible element is a pack of thin metal discs. A cracked disc or loose connecting bolts can shift the pump's position relative to the engine, and so the injection timing: knock, smoke, power loss and hard starting can result. If the coupling breaks, the pump stops turning and the engine stalls.
After a cracked-disc or loose injection pump drive coupling is replaced, the injection timing is set to the marks and values in the service documentation; wrong timing can damage the engine. On unit pump, unit injector and common rail engines, the engine control unit sets injection timing, and the coupling-and-timing description does not apply to these systems.
| Part | Function | Seen when it fails | At replacement |
|---|---|---|---|
| Overflow valve | Holds supply pressure, sends excess diesel and air to the return | Hard starting, power loss, fitting leak | Same opening pressure and OEM number, new washers |
| Coupling | Connects the pump to the drive; on adjustable types allows timing adjustment | Knock, smoke, power loss; if it breaks, the engine stops | Timing is reset, bolts tightened to the specified torque |
| Pump repair kit (gaskets, seals, washers) | Housing, cover and shaft sealing | Diesel or oil leak to the outside | A kit that fits the pump type; internal parts are bench work |
| Injection line | Carries diesel from the pump to the injector at high pressure | Fitting leak, vibration crack | Correct length and bend, clamps, the manufacturer's tightening value |
How is an injection pump problem fixed, and when should it go to a shop?
External parts are replaced in the workshop; the inside of the pump needs measurement on a test bench. Tightening values, injection timing and bleeding for washer, gasket, overflow valve, coupling and line replacement follow the manufacturer's service documentation.
For internal problems such as pumping element wear or a governor or control rack fault, the decision to overhaul or replace is made after measurement at a competent diesel shop. On a common rail system, the fault code, rail pressure and supply side are checked before a pump is replaced. Dirty or water-contaminated diesel damages the pump's internal parts: water reduces the lubricity of diesel and can seize close-tolerance parts. If the fuel and filters are not corrected, the renewed pump will fail too.
See a shop without waiting if diesel is getting into the engine oil, if a leak is increasing quickly or if the truck stalls while driving.
When choosing parts, use the type and part number on the pump's label, the OEM number, the engine code, the vehicle identification number (VIN) and the model year together. Injection pump parts that are replaced from outside the pump, such as the overflow valve, coupling, repair kit and lines, are matched by OEM number; the method is explained in the article on finding the right spare part with an OEM number. The search in the MPT catalog can also be done by OEM number.
Before working on the fuel system, secure the truck on level ground with the parking brake and chocks, follow the manufacturer's instructions if the cab is to be tilted, let the engine cool, work away from open flames, and clean up spilled diesel immediately.