Cummins ISX11.9 CM2250 – Service Manual 005-999   Fuel System – Overview

The XPI fuel system is a high-pressure common rail injection system.  The fuel system is made up of two smaller primary systems, a low-pressure system and a high-pressure system.  The components upstream of the high-pressure fuel pump compose the low-pressure system and the high-pressure fuel pump and the components downstream of the high-pressure fuel pump compose the high-pressure system.

Vehicle plumbing connects the fuel tank to an OEM mounted 10-micron, suction side filter/water separator, then to the engine mounted fuel filter head.

There are three components that provide or receive input to or from the electronic control module (ECM).  The normally open fuel pump actuator receives a pulse width modulated (PWM) signal from the ECM to open or close, in response to the signal from the fuel rail pressure sensor. The injectors have individual solenoids. The ECM powers each injector individually to provide fueling to each cylinder.

If equipped, an optional fuel lift pump will assist in priming the fuel pump gear pump. The fuel lift pump runs for approximately 60 seconds after key ON.

Fuel flows through the fuel lift pump, if equipped, to the fuel pump gear pump, then through a 5-micron, pressure-side filter. Filtered fuel is then fed to the high-pressure fuel pump head.

The high-pressure fuel pump head contains an air-bleed fitting and the fuel pump actuator. The air-bleed orifice fitting in the fuel pump head aids in purging air from the fuel supply. Because of the air-bleed orifice fitting, some fuel that is supplied by the gear pump will return to the drain at all times.

The high-pressure fuel pump is driven at crankshaft speed by a fuel pump idler gear. The gear pump is driven by the pump camshaft through an internal coupling.

The fuel pump actuator is opened or closed by the ECM, to maintain the appropriate fuel rail pressure. Fuel that is metered past the fuel pump actuator enters the high-pressure fuel pump inlet drilling, moves past the inlet check valve, and fills the pumping chamber by pressing the pumping plunger downward. When the camshaft pushes the pumping plunger upward, fuel reaches rail pressure and causes the outlet check valve to lift. Fuel then enters the outlet drilling of the fuel pump and exits the high-pressure fuel line to the fuel rail.

The high-pressure fuel pump can be divided into three distinct assemblies. They are the fuel gear pump, cam housing, and high-pressure fuel pump head.

The high-pressure pump is driven by the engine camshaft. The gear pump is driven by the pump camshaft through an internal coupling.

Each of the two pumping plungers is driven by a three lobed camshaft. The camshaft is located in the cam housing module by tapered roller bearings. The bearings that support the camshaft, as well as the tappets, rollers, and camshaft itself are lubricated with engine oil. These are the only components in the pump lubricated with engine oil.

Engine oil to the high-pressure pump is supplied through a drilling in the engine rear gear housing. The oil passes from the engine rear gear housing to the high-pressure pump cam housing. A small o-ring in a recess on the back of the engine rear gear housing seals this passage.