A.M. Diesel

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Reconditioned Fuel Pumps of ALL Makes


Fault Finding


Specialist Fuel Injection Repair



ECU Repair and Installation



Injector (Common Rail)



Air Conditioning Service


General Info

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Injector Operation

Each pumping plunger moves up and down under the action of the engine camshaft via a rocking lever and a plunger return spring. On the upward (filling) stroke, fuel at feed pressure enters the chamber below the plunger via the cylinder head fuel feed passage, non return valve, fuel passage 'A', the open spill control valve and fuel passage 'B'. During the downward (pumping) stroke, except for the period that the spill control valve is closed to enable injection, fuel passage 'A' and the cylinder head fuel feed passage.

When the stator is energised, it attracts the armature, and thus the spill controll valve, which closes. Fuel pressure then increases and fuel passes via the fuel passage 'C' to the injector nozzle.The nozzle opens and fuel is sprayed into the combustion chamber. When the stator is de-energised, the spill control valve is opened, causing the pressure to decrease rapidly, allowing the injector nozzle to close and terminate injection.

Injection timing and fuel quantity are determined by the relationship between the plunger position and the electrical signal supplied to the stator by the ECU.

Electronic Pilot Injection/2 Stage Lift VCO Nozzle

The EUI (Electronic Unit Injector) features a combination of electronic pilot injection with a two stage lift VCO (Valve Covering Orifice) type nozzle to further optimise and refine the injectors performance on engine.
The use of pilot injection allows the EUI to deliver a small amount of fuel at a pre-determined point prior to the main charge. In the engine this has the effect of pre-conditioning the combustion process and hence significantly reducing the noise produced. The pilot fuel is injected, usually at engine idle speed, by simply energising the spill control valve twice during the downward stroke of the plunger.
The pilot injection feature is complemented bya two stage lift VCO type nozzle. This assembly is mechanically set to lift at pre-determined opening pressures in two stages. The first stage (pre-lift), is set at a relatively low opening pressure, which when combined with the nozzle valve tip geometry, lowers the initial rate at whic the fuel is injected. The second stage (full-lift), is set at a higher opening pressure to ensure adequate nozzle valve closing force upon injection termination. Lowering the initial rate of injection of the fuel benefits engine noise and emission levels.

Operating Principle

The fuel injection pump and injector are combined in a single unit. The EUIs (Electronic Unit Injectors) are situated in the cylinder head above the combustion chamber, with the pumping plungers driven by the engine camshaft via rocking levers. Fuel feed and vent are through passages in the cylinder head.

Sensors provide information to the ECU (Electronic Control Unit) on the relevant engine operating parameters which are typically:

  • accelerator pedal position                         

  • air temperature

  • coolant temperature

  • engine speed and timing

  • air pressure

  • air flow


This information is continuously compared with the optimum values stored in the ECU'S memory. The result of that comparison is translated into signals instrcting the unit injector solenoid-actuated spill control valve system to deliver the correctly metereds fuel at the timing point required by the engine.


Pressure Assisted Nozzle Closure

The EUI (Electronic Unit Injector) has a unique patented feature which has been developed to hydraulically assist the nozzle valve to close upon injection termination. This utilises the injector spill energy to provide additional force to close the valve faster than is possible with the nozzle springs acting alone. This feature ensures a very rapid termination of injection which further reduces engine emission levels. When the EUI is injecting fuel, the nozzle valve moves to its full lift stop awaiting the signal to terminate injection. To commence termination, the stator is de-energised and the spill control valve opens releasing the internal system pressure and allowing fuel to flow through fuel passage 'B', through the spill control valve and into fuel passage 'A'. The fuel is prevented from spilling directly back to the cylnder head fuel return passage by the none return valve. As the spill flow increases, the pressure backing piston is forced downwards driving the nozzle valve closed, terminating injection. When the spill force dissipates, the pressure backing piston returns to its orignal position awaiting the next injection cycle. An added benefit of this feature is that the mechanical refinement of the injector is greatly improved due to the spill flow pressure being released in a controlled manner.

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