HEUI fuels systems as found in International and Ford products.
“HEUI” stands for Hydraulic Electric Unit Injection, and is the system that International and CAT use on their electronic medium duty diesel engines from approximately 1994 thru 2006 and beyond in some I-6 engines.
The HEUI system uses engine oil, highly pressurized, to inject fuel into the cylinders. This is called the Injection Control Pressure (ICP) system. The pressure of this high pressure oil is computer controlled, and regulated by the Injection Pressure Regulator (IPR) valve/solenoid. Each cylinder has one diesel fuel injector, and is supplied with High Pressure oil from a special pump - the High Pressure Oil Pump (HPOP). The injectors are controlled by the engine’s electronic computer, in some cases directly, and in others via an Injector Drive Module (IDM). The injectors have fuel supplied by a (relatively) low pressure fuel transfer pump, and this fuel is continually replenished in the injector as it is injected. The injectors work on the principal of hydraulic “intensification”, where the Injection Control Pressure (ICP) is multiplied inside the injector to attain fuel injection pressures as high as 31,000 PSI. This provides for high fuel system efficiency and excellent atomization of the fuel.
Overivew of the ICP system:
The HPOP gets oil from the main engine oil pump (unfiltered in the 7.3L), and pressurizes it based on engine RPM and load. The control for the ICP pressure is the IPR valve, and the amount of ICP directly affects the fuel injection pressure due to the intensifier inside the fuel injectors. Engine oil is supplied from the oil pump to the HPOP at regular engine pressure. The oil enters the HPOP from the oil reservoir. On the 7.3L engines this is mounted “front and center”, whereas on the 6.0L it is underneath the oil cooler assembly where a screen keeps out the trash. With either the 7.3L or the 6.0L the HPOP is a low volume, but high pressure capable pump. This means that it can generate 5,000 psi and greater, but only if the ICP system is mostly leak-free.
Oil leaving the HPOP and traveling into the IPR valve which is screwed into the end of the HPOP. This oil has only two possible paths; out to the injectors, or back to the engine oil pan. The output pressure of the HPOP is controlled directly by the IPR valve which acts like an electrically (PCM) controlled pressure relief valve. With 12 volts on the IPR valve power wire, but no “duty cycle” on the IPR ground wire, the valve is “wide open”, and DRAINS most of the oil that the HPOP sends to the valve back into the engine oil pan. With the IPR valve at 100% duty cycle (grounded 100%), the ICP pressure is maximized as no oil can flow back to the pan. The IPR duty cycle is the amount of time the valve is CLOSED (no oil draining to the oil pan). It is NOT the percentage closed it is, like a traditional faucet. The IPR valve is either completely open, or completely closed. So the “duty cycle” of the IPR valve represents only the percentage of TIME it is closed.
High Pressure oil leaving the HPOP/IPR assembly is headed to the injectors. In the 7.3L engine, there are two High Pressure hoses, one for each cylinder bank that carry the oil to a cast-in-place manifold in the cylinder head. The oil manifold has drillings that allow the HP oil to flow to the injectors, where it enters thru holes in the side of each injector. The fuel and HP oil are separated by special o-rings that can withstand the very high hydraulic pressures of the ICP system. If this o-ring fails, you get engine oil loss, and it shows up in the fuel system as “black fuel”. Normally, this is the only place the oil in the ICP system where leaks (pressure loss) occurs. In the 6.0L engine, the HP oil leaves the pump assembly thru a Snap To Connect (STC) fitting where it then travels into the “branch tube” that supplies each bank of the engine with HP oil via the “Case to Head Tubes”. These tubes run from inside the “V” of the engine up into the cylinder head where they fit to the Under Valve-Cover oil Manifold (UVCM). From the UVCM, each injector gets it’s oil directly thru the top of the injector. In this system, it is very rare to have engine oil contaminating the fuel, in fact the only way this can happen is if the injector is internally failed. In both engines, an “Intensifier” (internal part of the fuel injector) effectively multiplies the ICP by a factor of 7, so the resulting fuel injection pressure is 7 times greater than that of the ICP system at the time. The most simple common example of how this works is a hydraulic ram. If you put 1,000 PSI in, and the area is 7 in^2 (7 sq in), you get 7,000 pounds of force out.
The vehicle’s engine computer (PCM or Powertrain Control Module in Ford speak) is programmed with a given fuel rate, timing, and IPR duty cycle for each engine condition. The fuel rate depends on throttle position, engine RPM, engine air flow (boost is how this is guestimated), and altitude (Barometric pressure). Fuel rate is affected by ICP. Higher ICP will inject fuel more quickly, or said another way it will inject fuel faster. Higher ICP is only needed and beneficial with increased cylinder pressures (power output). The HPOP takes power to turn it, and making more ICP than needed, simply wastes power that would otherwise go to the wheels. The PCM either fires the injectors itself, or sends the timing and fuel rate commands to the IDM for it to do. In the 7.3L engine, the IDM can only turn the injectors “ON”, so it must maintain the “ON” power levels for the entire time fuel is injected. Since this system uses as much as 115 volts, IDM failure is relatively common. These “single coil” injectors, were used on all non-EGR equipped International engines, and most CAT engines as well. They have proven to be very reliable and durable, but with time and mileage often benefit from a “tune up” where the injector coil is re-shimmed to “as new” specifications greatly improving cold start-up and running. The injectors used in the EGR equipped 6.0L (VT365 International branded engines) are called “G2” injectors, for “Generation 2”. These injectors are operated at significantly lower voltages than the single coil units, at 48 volts, so the voltages are not considered “dangerous”. They also feature TWO coils, one to open the injector’s spool valve to begin fuel injection, and the other to close the spool valve to stop fuel injection. Because these injectors have two coils, and due to their design, it is not necessary to keep the coils energized, they receive only a brief “pulse” of power from the IDM at the appropriate time. This makes the IDM much more energy efficient and reduces the likelihood of failures. The “G2” injectors also respond much faster to the IDM so fuel control is much improved for better power and superior emissions. When the IDM powers the injector’s “open” coil, the spool valve allows HP oil to flow into the injector, thereby injecting fuel while the spool valve is open. The spool valve remains open due to residual magnetism in the open coil. When the IDM has determined the injector has been on long enough, it powers the “close” coil, and the spool valve moves to the closed position preventing additional HP oil from entering the injector. The spool valve also allows HP oil to drain from the intensifier during the time it is closed. When the engine is shut off, the IDM closes ALL the injector spool valves to shut off fuel delivery.
Both the 7.3L and the 6.0L have external fuel transfer pumps that supply fuel to the injectors at approximately 60 psi. The 7.3L is not too fussy about fuel pressure, the 6.0L however is very picky, and low fuel pressure can cause severe injector damage and poor drivability. That is not to say that there is any benefit from increasing the factory pre-set fuel pressure on a 6.0L, it is more important to ensure that the regulator is working correctly, and that the fuel system’s filters are serviced as required. The external filter, mounted in the fuel module on the LH frame, is the more important of the two filters to keep an eye on. The main filter, mounted on the engine, is pressurized, and somewhat less prone to causing troubles.
Due to the fact that the HEUI system uses engine oil to power the injection of fuel, proper oil changes, oil viscosity, and care are VERY important to proper system operation. As engine oil warms, it thins out, so the PCM adjusts ICP based on engine oil temperature. Since there is no way to effectively measure the viscosity of the engine oil during operation, the PCM is pre-programmed with what ICP goes with a given oil temperature. As we know, oil viscosity is also affected by the weight of the oil, and to some extent it’s quality. To ensure proper injector function, and engine operation, it is CRITICAL that only oils carrying an API CI-4 (1994-2006) or CJ-4 rating, and recommended oil viscosities (15W-40 / 10W-30) be used. Oils having “unusual” viscosity ranges (like 5W-40) are generally not recommended as they can adversely affect injector function by causing abnormal injection timing. DO NOT use oil for gasoline engines!
'93 F-250 HD, 7.3L IDI, 5spd - FARM TRUCK
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