864689 Mercruiser PLUG

Illustration 1 g00923453
Typical fuel system schematic
(1) Fuel supply line
(2) Unit injectors
(3) Fuel gallery (fuel manifold)
(4) Electronic Control Module (ECM)
(5) Pressure regulating valve
(6) Secondary fuel filter
(7) Fuel priming pump
(8) Distribution block
(9) Fuel temperature sensor
(10) Fuel transfer pump
(11) Pressure relief valve
(12) Check valve
(13) Fuel tank The fuel supply circuit is a conventional design for unit injector diesel engines. The system consists of the following major components that are used to deliver low pressure fuel to the unit injectors:Fuel tank - The fuel tank is used to store the fuel.Fuel priming pump - The fuel priming pump is used to evacuate the air from the fuel system. As the air is removed the system fills with fuel.Fuel filter - The fuel filter is used to remove abrasive material and contamination from the fuel system.Supply lines and return lines - Supply lines and return lines are used to deliver the fuel to the different components.The purpose of the low pressure fuel supply circuit is to supply fuel that has been filtered to the fuel injectors at a rate that is constant and a pressure that is constant. The fuel system is also utilized to cool components such as the electronic control module and the fuel injectors.Once the injectors receive the low pressure fuel, the fuel is pressurized again before the fuel is injected into the cylinder.The unit injector uses mechanical energy that is provided by the camshaft to achieve pressures that can be in excess of 200,000 kPa (30,000 psi).Control of the fuel delivery is managed by the engine's Electronic Control Module (ECM). Data from several of the engine systems is collected by the ECM and processed in order to manage these aspects of fuel injection control:
Injection timing
Fuel injection timing advance
Injection duration
Engine cold mode status The mechanical electronic fuel system relies on a large amount of data from the other engine systems. The data that is collected by the ECM will be used in order to provide optimum performance of the engine.Low Pressure Fuel Supply Circuit
The flow of fuel through the system begins at fuel tank (13). Fuel is pulled from the tank by fuel transfer pump (10). The fuel transfer pump incorporates a check valve (12) that will allow fuel to flow around the gears of the pump during hand priming of the fuel system. The fuel transfer pump also incorporates a pressure relief valve (11). The pressure relief valve is used in order to protect the fuel system from extreme pressure.The fuel transfer pump is engineered in order to produce an excess fuel flow throughout the fuel system. The excess fuel flow is used by the system to cool the fuel system components. The excess fuel flow also purges any air from the fuel system during operation. Air that can become trapped in the fuel system can cause cavitation that may damage the components of the unit injector.The fuel travels from the fuel transfer pump to distribution block (8). A fuel temperature sensor (9) that is installed in the distribution block is used to sample the fuel temperature. A signal that represents the fuel temperature is sent to the electronic control module (ECM) for processing.The fuel is then pumped in to the fuel filter base. In most applications, fuel priming pump (7) is located on the fuel filter base. The fuel filter base also incorporates a siphon break that prevents fuel from draining from the fuel system when the engine is not in operation. The priming pump utilizes a series of check valves in order to direct the flow of fuel during the priming pump's operation. The check valves work with the fuel priming pump in order to produce a pumping action. The check valves also prevent fuel from being forced back into the fuel transfer pump. The fuel flows through a two micron fuel filter (6). The filtered fuel then flows out of the fuel filter base.If a fuel cooled ECM is installed on the engine, the fuel is pumped into the ECM. The fuel travels through the cored passages of the electronic control module housing in order to cool the control module's electronics.The fuel is transfered by fuel supply lines (1) to fuel gallery (3) in the cylinder head or to fuel manifold (3). Only a portion of the fuel that is supplied to the fuel injectors is used for engine operation.The fuel that is unused by the engine is provided for cooling purposes. This unused fuel is discharged into the return passages of the fuel gallery. The fuel is returned to the fuel tank by the fuel return lines. A continuous flow of fuel is experienced within the low pressure fuel system.During engine operation, fuel injectors (2) receive fuel from the low pressure fuel system. The injector pressurizes the fuel to high pressure. The fuel is then injected into the cylinder. The excess fuel is returned to the tank. Refer to Systems Operation, "Unit Injector" for a complete explanation of the injection process.A pressure regulating valve (5) is located in the fuel return. The pressure regulating valve allows the low pressure fuel system to maintain a constant pressure. A flow control orifice is also located in the fuel return. The flow control orifice maintains a system back pressure that is constant. The orifice allows the flow of fuel through the system to be constant. This prevents excessive heating of the fuel.Fuel Heaters
Fuel heaters help to prevent the plugging of the fuel filters in cold weather. This plugging is called waxing. In cold ambient conditions, the cold engine does not dissipate enough heat into the fuel system in order to prevent waxing. Heaters that are not thermostatically controlled can heat the fuel in excess of 65° C (149° F). Excessive temperatures in the fuel system will drastically reduce the efficiency of the engine. The fuel system's reliability is also affected by high fuel temperatures.Note: Never use fuel heaters without some type of temperature regulator. Ensure that fuel heaters are turned OFF during warm weather conditions.Fuel System Electronic Control Circuit
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