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  • Common Rail Diesel

    The  common rail diesel injection system was developed by Bosch . Common rail diesel engines can be found in many modern vehicles including cars, vans, buses, trucks, and earthmovers.

    The common rail diesel fuel system is an electro-mechanical system for control of fluid (fuel) pressure and flow.  Diesel fuel must be  delivered at the correct moment  and atomised in a fine spray to form an air fuel mixture inside the engine combustion chambers  to allow efficient burning . There are several manufacturers of common rail diesel fuel injection systems including Bosch , Delphi ,  Nippondenso , Siemens.

    Advantages of the Common Rail Diesel System

    The electronically controlled common rail system has many advantages:

  • Higher Performance through increased torque at low engine speed
  • Lower fuel consumption
  • Lower soot emissions - use of diesel particulate filter ( dpf ) in exhaust system.
  • Reduced Noise
  • Closed loop control
  • Programmable characteristics enabling effective use of biofuels
  • Electronic diagnostics
  • Diesel Lambda Control using exhaust gas recirculation to reduce NOx emission

    The above diagram shows a very basic layout of a typical common rail diesel fuel system. Electronic sensors and actuators not shown . Back leak rate comparison test of common rail diesel injectors on Bosch CP1 system fitted to 5 cylinder diesel engine.


    General System Overview

    Diesel fuel is drawn from the fuel tank by a low pressure transfer pump, this may be an electric pump located in the fuel tank or mechanical pump feeding the  main high pressure fuel  pump. On its journey to the high pressure pump the fuel will pass through at least one fuel filter assembly.
    Fuel at a constant transfer pressure is then passed to the intake of the mechanical high pressure pump which acts to increase fuel pressure to a minimum of approx 220 bar during cranking,  to around 1600 bar  (approx 23000psi) operating pressure whilst the engine is running at full load. Newer systems are capable of operation at higher fuel pressure.
    The pressure at the high pressure pump is regulated by  controlling the quantity of fuel entering the pumping elements within the high pressure pump or by control of the amount of pressurised fuel returning from the fuel rail to the tank.
    Lubrication of the pump is provided by fuel circulation.
    The highly pressurised fuel is pumped to the common rail   . The  fuel accumulated in the common rail is delivered to each engine cylinder by an electronically controlled  fuel injector.  A fuel rail pressure sensor is used to provide  a fuel pressure feedback signal  to the engine controller.
    Each injector is electronically controlled allowing precise operation to deliver the required amount of fuel at the correct time. Common rail diesel fuel systems with electromagnetic solenoid valve type injectors allow a degree of fuel to return to the tank by internal leakage through the injector body, referred to as injector back leakage.
    Excessive back leakage of fuel at the fuel injectors due to wear can cause a drop in common rail fuel pressure and in some circumstances may prevent the vehicle engine from being started.

    Regular servicing is essential to prevent fuel flow restriction, contamination of precision components and consequent engine running problems. Fuel quality is of paramount importance due to close tolerance of machined  precision system components. Misfuelling of a diesel vehicle with another fuel such as petrol can cause extensive damage. Certain vehicle insurers are now offering insurance policies protecting against misfuelling.

    Safety Note

    Common rail diesel fuel systems operate at very high pressure and can cause severe injury. Fuel pressures of up to 2200bar may be present. Please refer to manufacturers health, safety and training provisions before attempting service or repair of any common rail or pressurised fuel system. We only recommend servicing by qualified personnel.


    Fuel Injector Calibration (Coding)

    Common fault codes:  P2336 , P2337 , P2338

    The amount of fuel injected is proportional to the injector duration (opening time) , fuel rail pressure , fuel temperature  & fluid viscosity.
    A target value for injector duration under specific conditions is programmed into the diesel control unit (DCU) engine control map.
    The fuel injectors are machined to very fine tolerances but because of individual characteristics such as friction, pressure drop, magnetic force , wear during service etc,  very slight variations in flow can occur.
    Slight difference between target flow and actual flow can occur due to high operating pressures.
    Certain common rail diesel fuel injectors require calibration during manufacture or overhaul  on a specialised diesel injection  test rig  to produce  a data code relating to the characteristics of each individual injector.  This data code may be referred to as the C2i code or IMA code depending on manufacturer.
    The data code is programmed into the engine control unit identifying to which cylinder the injector is fitted.
    This data code allows the engine control module to correct the pulse duration applied to each individual injector to maintain optimum performance. Fitment of replacement injectors will require the new injectors to be coded to the engine control unit. We  have facilities for injector coding of several makes of vehicle.

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    Common Rail Fuel System Faults

    Typical Fault Possible effect Possible Cause
    High Pressure Fuel System :
    Loss of  Rail pressure
    Difficult staring

    Non Start

    Engine cuts out

    Excessive exhaust emissions


    Control unit reverts to limp home mode

    Rail pressure sensor signal out of expected range


    Worn fuel injectors with excessive back leak rate.

    Worn or defective fuel pump

    Too slow engine cranking speed

    Fuel pressure regulator,IMV,MPROP, wiring, etc




    Low pressure Fuel delivery Difficult Starting

    Non start

    Engine cuts out

    Pump related fault codes

    High pressure fuel system faults

    Blocked or damaged fuel supply lines.

    Fuel filters restricted by blockage

    Electric fuel pump

    Low pressure gear pump


    Carbon build up around fuel injectors Loss of cylinder pressure Loss of seal between fuel injector and cylinder head. may require injectors removed and specialist machining to cylinder head
    Air intake system faults Non start

    Difficult starting


    Poor performance under load

    Restricted air filter

    MAF (Mass Air Flow sensor) fault

    Loss of  Turbocharger boost pressure

    Exhaust gas recirculation system faults

    Exhaust system faults Non start

    Difficult starting

    Loss of power

    EGR system restricted , faulty , or inoperative.

    Blocked or restricted Diesel particulate filter (dpf)


    Electrical and electronic faults Non start


    Engine cuts out

    Broken wiring

    Damaged wiring insulation

    Corroded connections

    Low battery voltage

    Electronic control system faults with Sensors ,Actuators, ECU, coding.

    Mechanical faults causing electronic system faults.

    Faults with the common rail fuel system often result in loss of  fuel pressure causing the engine to cut out .

  • Fuel Quality resulting in contamination of filters causing flow restriction.
  • Incorrect Fuel used causing damage to system components - particularly when a driver has accidentally refuelled with petrol instead of diesel.
  • Electrical wiring , Sensor and Actuator faults
  • Most common rail fuel systems use a fuel accumulator rail mounted pressure sensor which will provide an electrical  feedback signal to the engine management controller.
    A loss of fuel pressure or a deviation from the desired amount of fuel pressure may be interpreted as a fuel system leak by the engine management controller in which case total system shut down may occur as a safety feature . Certain electronic management systems can lock out and prevent engine start until faults have been repaired.
  • Faults caused by Exhaust gas recirculation resulting in excessive carbon build up and blockage of intake components.

    Typical example of a vehicle suffering from engine running fault

    The vehicle in question was a late model luxury car fitted with a 6 cylinder common rail diesel engine which had covered around 90,000 miles. The customer reported an intermittent fault which caused sluggish acceleration and rough engine running was sometimes present. Unusually,the dashboard engine management light , in this particular instance was not illuminated. The customer had already tried to locate the fault by unplugging and reconnecting wiring to various engine components.
    A visual check of the engine bay was done, then using the diagnostic scantool we interrogated the engine control module for stored fault codes. Several fault codes relating to different fuel injectors, engine stability, and P3505 pre heat device faults were present.
    Fault codes were recorded ,then cleared from engine control unit memory. The vehicle was then road tested by accompanying the customer.
    On road test the vehicle hesitated breifly during acceleration. After road test the control unit memory was once again read using the diagnostic scanner.
    A P0204 cylinder fault code relating to an electrical fault within the circuit to cylinder 2 injector valve was present. Moving the wiring to cylinder 2 injector located this fault to a bad connection at the injector electrical connector plug , noted by the audible change in engine note. The bad connection was rectified and fault codes once again cleared.
    A back leakage test was carried out of all six injectors with the engine running. Five of the injectors had back leakage values within 10% of each other. However the remaining fuel injector had almost twice the back leakage fuel flow in comparison to the others indicating a badly worn fuel injector. Injectors were removed from the engine for test bench assessment of spray pattern ,recondition and calibration.
    The serviced injectors were refitted together with new high pressure pipes and new fuel filters as recommended and road test carried out resulting in a much smoother drive.

    Electronic diagnostics
     The common rail diesel fuel injection system is operated and monitored by an electronic control unit using closed loop control strategy.  If a problem occurs with system operation a fault error code may be generated and stored in engine control unit memory.  The control unit is programmed to take appropriate action and may alert  the driver by causing the engine management light MIL to illuminate and also by reducing fuel rail pressure to restrict engine performance to a limp home function or under certain conditions full shutdown of the injection system.

    Logical  system diagnostics are  first carried out using a scantool with dedicated software to read fault codes which may be stored on engine management computer. Live data may be used to monitor fuel rail pressure as seen by engine control ecu.

    More advanced diagnostics can be carried out on individual  electronic components by use of oscilloscope testing.




    Tel: 07703 558610 for Auto electrical & Diesel car mobile breakdown service in Telford & Shrewsbury , Shropshire areas.