HEUI FUEL SYSTEM OPERATION - Continued
extends to the back of the cylinder head. This passage connects with each unit injector bore to supply fuel to unit
injectors. Fuel from the transfer pump flows through the cylinder head to all of the unit injectors. Excess fuel
flows out of the back of the cylinder head and into the fuel pressure regulator. The fuel pressure regulator is an
orifice and a spring loaded check valve. The orifice restricts flow to pressurize the supply fuel. The spring loaded
check valve opens at 35 kPa (5 psi) to allow the fuel which has flowed through the orifice to return to the fuel tank.
When the engine is off and no fuel pressure is present, the spring loaded check valve closes. The spring-loaded
check valve closes to prevent the fuel in the cylinder head from draining back to the fuel tank.
Injection Actuation System
The injection actuation system supplies high-pressure oil to power HEUI injectors and controls the injection
pressure produced by the unit injectors by changing the actuation pressure of the oil. Oil from the engine oil
pump supplies the needs of the engine lubrication system and the hydraulic injection pump for the fuel system.
Oil drawn from the engine sump is pressurized to the lubrication system oil pressure by the engine oil pump. Oil
flows from the engine oil pump through engine oil cooler, through the engine oil filter and then to the main oil
gallery. A separate circuit from the main oil gallery directs a portion of the lubrication oil to supply the injection
Oil flows into the inlet port of injection pump and the oil fills the pump reservoir. Oil from the pump reservoir is
pressurized in the hydraulic pump and pushed out of the outlet port under high pressure. Oil then flows from the
to the high-pressure oil passage in the cylinder head.
The high-pressure oil passage connects with each unit injector bore to supply high-pressure actuation oil to unit
injectors. Oil is contained in the high-pressure oil passage until used by the unit injectors. Oil that has been
exhausted by the unit injectors is expelled under the valve covers. This oil returns to the crankcase through oil
drain holes in the cylinder head. Under most operating conditions, the injection pump is producing excess flow
that must be discharged to a drain to control the system pressure. The IAP control valve regulates system
pressure by discharging the precise amount of oil to the drain.
The ECM selects the desired actuation pressure based on sensor inputs and performance maps. The ECM
sends a control current to the IAP control valve to change the actual actuation pressure. The IAP control valve
reacts to the electrical current from the ECM to change the actual actuation pressure by discharging pump flow to
the drain. The IAP control valve is an electrically controlled relief valve. The IAP sensor monitors the actual
actuation pressure in the high-pressure oil passage and reports by sending a signal voltage to the ECM. The
injection actuation pressure control system operates in a cycle. ECM selects the desired actuation pressure and
sends an electrical current to the IAP control valve that should produce that pressure. The IAP control valve
changes the pressure relief setting, which in turn changes the actual actuation pressure. The IAP sensor
monitors the actual actuation pressure and sends a signal voltage back to the ECM. The ECM interprets
the signal voltage to calculate the actual actuation pressure and compares the actual actuation pressure to the
desired actuation pressure to adjust the electrical current to the IAP control valve. The IAP control valve
responds by changing the actual actuation pressure. This closed loop control system is repeated 67 times per
Most of the high pressure oil flow from injection pump is used to power unit injectors. Excess flow is returned to
the drain through the IAP control valve. The excess flow travels through a drilled passage to the front of the
pump. Drain oil flows out of the front of the pump over the pump drive gear and flows down the engine front gear
train to the sump.