TM 9-6115-729-24
0003
HYDRAULIC ELECTRONIC UNIT INJECTOR (HEUI) FUEL SYSTEM - Continued
detected, a diagnostic code is generated and sent to the GSC where it is displayed. In most cases, the code is
also logged with the engine hour value indicating when the failure occurred. When diagnostic codes are
generated, they are referred to as active. They indicate a problem presently exists. These should be serviced
first. Diagnostic codes stored in memory are referred to as logged failures. The existence of a logged code does
not indicate a current failure. The failure may have occurred some time ago. The logged codes are useful when
troubleshooting an intermittent problem. Logged inactive codes require a generic password to clear as described
in WP 0004 and WP 0085. Codes not requiring a password are automatically deleted from memory after 100
engine operating hours.
Hydraulic Pump
The hydraulic injection pump (high-pressure oil pump) is located at the left front corner of the engine. The
hydraulic pump is a fixed displacement axial piston pump. The hydraulic pump uses a portion of the engine
lubrication oil pressurized to the injection actuation pressure required to power the HEUI injectors.
Injection Actuation Pressure Control Valve (IAP Control Valve)
The injection actuation pressure (IAP) control valve is located on the side of hydraulic pump. Under most
conditions, the pump produces excess oil flow. The IAP Control Valve discharges excess pump flow to the drain
to control injection actuation pressure to the desired level. The performance maps of ECM contain a desired
actuation pressure for every engine operating condition. The ECM sends a control current to the IAP control
valve. The control current should make the actual actuation pressure equal to the desired actuation pressure.
The IAP control valve is an actuator that converts an electrical signal from ECM to the mechanical control of a
spool valve that controls pump outlet pressure.
Fuel Transfer Pump
The fuel transfer pump is mounted on the back of hydraulic pump. The fuel transfer pump draws fuel from the
fuel tank. The fuel transfer pump pressurizes the fuel to 450 kPa (65 psi). The pressurized fuel is supplied to
injectors. The fuel transfer pump is a spring-loaded, single piston pump. The pump is operated by an off-center
bearing on the back of the hydraulic pump shaft. There are two check valves in the fuel transfer pump. The inlet
check valve opens to allow fuel from the tank into the pump and closes to prevent fuel leakage back to the
fuel tank. The outlet check valve opens to supply fuel to the fuel supply passage in the cylinder head. The fuel
supply passage supplies fuel to injectors. The outlet check valve closes to prevent pressurized fuel leakage back
through the pump.
Injection Actuation Pressure Sensor (IAP)
IAP sensor is installed in the high-pressure oil manifold. The high-pressure oil manifold supplies actuation oil to
power the unit injectors. The IAP sensor monitors injection actuation pressure and sends a continuous voltage
signal back to ECM.
HEUI FUEL SYSTEM OPERATION
Low Pressure Fuel System
The low pressure fuel system supplies fuel for combustion to injectors, supplies excess fuel flow to cool the unit
and to remove air from the system. The low-pressure fuel system consists of fuel tank, filter, transfer pump and
pressure regulator. The fuel transfer pump pushes pressurized fuel out of the outlet port and draws new fuel into
the inlet port from the fuel tank. Fuel flows from the outlet port of fuel transfer pump to the fuel supply passage in
the cylinder head. The fuel supply passage is a drilled hole that begins at the front of the cylinder head and
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