The 3126 HEUI fuel system uses hydraulic electronic unit injectors (3) . Hydraulic electronic unit injectors use high pressure oil to pressurize the unit injector.
The fuel supply circuit is a conventional design for injected engines. The circuit uses a transfer pump (8) with a single piston in order to circulate fuel at low pressure. The fuel flows at low pressure through fuel filter (7) into the cylinder head fuel passages. Fuel is delivered at high pressure into the combustion chamber through hydraulic electronic unit injectors (3) . The unit injectors are located near the center of the combustion chambers in the cylinder head between the rocker arms.
Transfer pump (8) draws fuel from fuel tank (17) . The fuel is typically filtered by a primary filter of approximately thirty micron rating, that is installed by the OEM. Some OEM’S may install a primary filter/water separator. The fuel transfer pump incorporates a check valve that permits fuel flow for hand priming. The fuel transfer pump also has a pressure regulating valve in order to protect the fuel system from extreme pressure.
From the fuel transfer pump (8) , fuel passes through a secondary fuel filter (7) of two micron rating. The fuel flows through the fuel filter into the fitting for the fuel manifold, that is located at the front of the engine. Fuel flows continuously around each unit injector (3) through an internal passage that runs the length of the head. Excess fuel exits at the rear of the engine through a pressure regulator in order to return to fuel tank (17) . Excess fuel that is not being used for injection aids in cooling the unit injectors and aids in purging air from the system. During normal under load operating conditions, the fuel pressure is maintained in the range of 400 kPa (58 psi) to 525 kPa (76 psi) by an orificed pressure regulating valve. The pressure regulating valve is located at the rear of the cylinder head. The pressure regulating valve also contains a check valve that serves as a siphon break. Fuel pressure at low idle should be a minimum of 500 kPa (73 psi) .
High pressure oil manifold (1) sends high pressure oil to unit injector (3) . When ECM (5) activates each unit injector the high pressure oil acts on a piston in the upper part of the unit injector. The piston has a surface area that is six times the surface area of the unit injector plunger. This larger surface area provides a multiplication of force. This multiplication of force enables the injectors to inject fuel into the cylinder at pressures higher than 140 MPa (20,000 psi) .
ECM (5) is located at the rear on the left side of the engine. The system provides total electronic control of the start and duration of the fuel injection. The ECM uses data from several sensors in order to adjust the fuel delivery. The adjustments are based on the performance map that is used in the software for engine control.
High pressure oil pump (15) is located at the left front corner of the engine. The high pressure oil pump is a gear-driven axial piston pump. Engine oil at normal pressure enters the high pressure oil pump. The high pressure oil pump increases the oil pressure to the actuation pressure that is required by unit injectors (3) .
Injection actuation pressure control valve (10) is located in high pressure oil pump (15) at the top left of the engine. Operational maps that are stored in the memory identify the optimum oil pressure for the best performance.
A problem with the components that send fuel to the engine can cause low fuel pressure. This can decrease engine performance