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Unlocking Efficiency: The Fuel Injection Pump Explained

By R&D
Published on April 5th, 2024

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The fuel injection pump applies high pressure – as high as 25,000 pounds per square inch (psi) - to fuel delivered to the engine’s fuel injectors. Depending on the engine management system employed and the specific pump design, the fuel injection pump may also determine the timing and amount of fuel delivered to the engine, thereby controlling engine timing and engine speed.

Diesel fuel injection pump

Fuel injection pumps are used on diesel engines. There are three types: in-line, rotary or rotary distributor, and radial. Radial pumps are sometime termed high pressure pumps. (Direct injection gasoline engines employ high pressure pumps that perform a similar function.)


The fuel injection pump is mounted on the engine itself. In-line and rotary pumps are usually located on top of the engine. On a V-8 engine, the pump may be located in the valley between cylinder banks. These injection pumps can be located by tracing back from the metal fuel lines leading to the individual cylinders. Radial pumps are mounted at the front of the engine, in the valley (General Motors Duramax) or behind the timing gear housing (Dodge Cummins).

Mechanical fuel injection pump showing the fuel lines


In-Line Fuel Pumps

In-line pumps were used on Dodge Rams with Cummins diesels from 1994 through 1998, and by many manufacturers in the 1960’s and 1970’s. Rotary pumps were the dominant fuel injection pump design for most manufacturers of diesel automobiles and light trucks from the 1980’s to the early 2000’s. Rotary pumps were manufactured either by Stanadyne (General Motors and Ford Power Stroke 6.9 and 7.3 liter diesels) or Bosch (Cummins powered Dodge Ram 1989 through 1993, 1998 through 2003). 

The Bosch CP3 is the dominant radial pump design, used by General Motors on the Duramax diesel until 2011, Dodge’s Cummins diesel from 2003, Jeep’s 2.8 liter diesel, and most late-model diesel automobiles. The newer Bosch CP4 radial pump is used on 2010 and later Ford 6.7 liter Power Stroke diesels and 2011 and later GM Duramax diesels.

An in-line pump has one cylinder - the “barrel” - for each engine cylinder. A piston - the “plunger” – moves up and down in each barrel. Fuel is pumped from the tank by the fuel pump to the injection pump, where it is admitted to the pressure chamber above the plunger through an opening - the inlet port - in the barrel wall. 

A camshaft with a lobe for each plunger is positioned under each barrel. As the camshaft rotates at half engine speed, it pushes each plunger upward in turn. As the plunger moves upward past the inlet, it closes off fuel inflow, pressurizing fuel in the chamber to expel it through an outflow valve that opens at a specified pressure.

An obliquely curved groove leading upward is cut into the side of plunger. As the cam lobe rotates away from the plunger and the plunger moves down, this grove passes the inlet port, allowing fuel to begin flowing through the grove into the pressure chamber. 

The bottom of the plunger is has gear teeth around its circumference, with the teeth in a gear rack. Moving the rack rotates all plungers within their barrels. This rotation alters the point along the oblique groove that first passes the inlet port, thereby regulating the total amount of fuel allowed into the pressure chamber before the plunger’s next upward stroke cuts off fuel intake.

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Rotary Fuel Pumps

A rotary pump employs only one chamber to pressurize fuel, which it then distributes individually to the engine cylinders. Fuel pumped by the fuel pump (sometimes termed the lift pump) is further pressurized by a transfer pump built into the fuel injection pump. 

The transfer pump increases fuel pressure proportionately to engine speed, delivering fuel to the fuel injection pump’s rotor, a solid cylindrical piece into which passages and cylindrical chambers have been machined. The rotor spins within a sleeve housing and is gear driven by the engine camshaft. 

To understand the rotor, visualize a soda can – but as a solid object, not a container. (A rotor is actually much smaller than a soda can.) Inside, an inch from the top, imagine there is a hollow chamber, with three or four large holes, equally spaced, drilled inward from the sides to connect to that hollow chamber. Put plungers in each of those holes, with the ends sticking out of the solid cylinder. 

Then machine small diameter passageways from the hollow chamber through the solid cylinder to the sides of the cylinder. As it rotates, the rotor’s inlet passage aligns with an inlet port in the housing wall and fuel flows through the inlet passage into the pressure chamber. This flow, along with the centrifugal force of the rotor’s rotation, pushes the plungers outward, filling the pressure chamber with fuel.

A fixed cam – like a collar with bumps on it – surrounds the rotor, with the plunger ends resting within the cam. As the rotor revolves, the plunger ends encounter the cam lobes and are simultaneously depressed. This pressurizes the fuel in the pressure chamber. 

As the rotor spins further, the rotor’s outlet passageway aligns with an outlet port in the housing leading to an individual cylinder’s fuel injector, where the pressure forces the injector open to spray fuel into the cylinder. This process is repeated, with each cylinder fed by its own individual outlet.

The amount of fuel admitted to the rotor is regulated by a control valve, determining engine speed. Timing is altered by rotating the fixed cam, to change the advance or retard the point in rotation at which the lobes depress the plungers. Cam advance is adjusted either by a mechanical advance mechanism or, in vehicles with electronic engine management, by electronic actuators.

Old engine repair fuel injection.

Radial Fuel Pumps

Radial fuel injection pumps pressurize fuel for delivery to the common rail that accumulates fuel for delivery to the injectors. Fuel is drawn from the tank, either by a fuel pump (or supply pump) or by a low pressure pump that is added as a component to the radial injection pump. 

Fuel is then metered through a control valve to three pumping chambers arranged radially around a rotating cam that has three equidistant lobes (CP3) or to two chambers around a cam with two lobes at 90 degree angles (CP4). 

As the cam rotates, the lobes simultaneously depress the plungers within the pressure chambers, pressurizing the fuel, which is then routed to the common rail where it is held at high pressure. The engine management system releases fuel from the common rail and controls timing and fuel delivery by opening and closing the fuel injectors.

Drivability Symptoms

Failure of in-line and rotary fuel injection pumps usually occurs gradually, marked by decreasing engine power and fuel economy. Hard starting is common in the late stages of injector pump failure and complete failure creates a no start condition. Engine roughness, missing or misfiring can also indicate a need for adjustment or replacement of the pump.

1998 to 2002 Dodge Rams with the Cummins diesel engine may also experience a dead, unresponsive accelerator pedal. If the pedal is released to the idle position for a few moments, pedal function will resume, until the condition reoccurs, worsening over time. 

On the Stanadyne DS4 fuel injection pump used on the 6.5 liter General Motors diesel, failure of the Fuel Solenoid Driver (FSD), also referred to as the Pump Mounted Driver (PMD), can cause the engine to stall and stop, though the engine will restart after the pump cools. The FSD/PMD is an electronic module that amplifies signals from the ECM to activate the solenoid powered control valve regulating fuel flow into the injection pump located on the side of the DS4 pump.

General Motors light trucks equipped with the Duramax diesel and 2003 and later Dodge Rams with the Cummins diesel, both of which use the Bosch CP3 radial fuel injection pump, may experience hard starting or surging at idle, both conditions caused by failure of the pump’s fuel pressure regulator. A failing radial fuel injection pump can also cause inadequate power under heavy load.

Inspection, Test, and Diagnosis

Car mechanic checking the fuel injector system

CAUTION: Fuel pressurized by a fuel injection pump, if released and contacting skin, will penetrate the skin. This can be fatal. Use extreme caution when inspecting a fuel injection pump or fuel lines to the injectors while the engine is running. Inspect for leaks with cardboard, not fingers.

Model year 1997 and later diesel powered automobiles and light trucks are equipped with On-Board Diagnostics, Generation 2 (OBD2). In these vehicles, generic Diagnostic Trouble Codes (DTCs) P0216 and P0251 through P0260 indicate injector pump malfunction. 

Additionally, each vehicle manufacturer designates enhanced OBD2 codes unique to that manufacturer that further refine diagnostic procedures. Vehicles manufactured before 1997 use manufacturer specific codes that can only indicate electrical continuity faults.

DTC P0216 indicates fuel injection pump timing does not match the timing commanded by the engine or powertrain control module. DTC P0216 typically results from incorrect adjustment of injection pump static timing, a defective timing control solenoid, sticking plungers, or injection pump failure, though there may be other causes (including air in the fuel system). 

DTC P0251 indicates a rotor or cam malfunction or fault in their electrical circuits. This code may also be set by running out of fuel. On General Motors light trucks equipped with the Stanadyne DS4 rotary pump, this code usually results from failure of the optical sensor mounted on the pump. That sensor provides pump speed, rotor position, cam position, and fuel temperature data to the Powertrain Control Module (PCM). DTC P0252 indicates the fuel control valve (or actuator) is not responding to command. DTCs P0253 through P0254 indicate the control valve signal is outside specifications.


Reprogramming is not necessary when replacing a fuel injection pump.


Regardless of design, the exacting close tolerances within a fuel injection pump make repair impractical. A failed fuel injection pump must be replaced with either a new or remanufactured pump. Procedures for removal and installation of a fuel injection pump differ, depending on pump type and vehicle manufacturer. 

It is usually necessary to remove a number of engine components to access the pump, which must then be disconnected from its fuel inlet and outlets. Removal of the pump often requires use of special service tools designed by the vehicle manufacturer for the procedure. Once the pump is installed, it must be primed and the air bled from the fuel system to and from the pump.

Repair of diesel fuel pump

Setting starting timing of a rotary fuel injection pumps is performed by loosening the pump body, rotating it slightly, and then retightening it. A Scan Tool may be required if the vehicle employs electronic engine management. 

The when installing a new pump, the flywheel must be in a specific position, or have been immobilized before the old pump was removed. This may require specialized tools, such as a barring tool that fits into the flywheel teeth of Cummins diesels or special service tools unique to the vehicle manufacturer.

Loss of power in a vehicle with a rotary fuel injection pump or an out of time P0216 trouble code can result from plungers sticking within their bores, diminishing fuel delivery. This can be caused by poor quality fuel. 

If a Stanadyne pump exhibits these symptoms, using the fuel additive manufactured by Stanadyne may cure the condition. If this code is caused on the Stanadyne DS4 by failure of the fuel solenoid driver/pump mounted driver (FSD/PMD), it may be replaced without replacing the pump itself. When replacing the FSD/PMD, the heat sink should be replaced or the pump relocated with an aftermarket kit to a remote location. This failure is usually caused by heat absorbed after engine shut down.

Failure of the Bosch VP44 rotary injection pump on the Dodge Cummins diesel is commonly precipitated by failure of the fuel pump (lift pump), forcing the transfer pump to draw fuel from the tank and lowering fuel pressure to the injection pump. This overheats the injection pump and its attached control module. On these vehicles, replacing the lift pump with an upgraded pump is recommended when replacing the VP44 injection pump.

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