HEUI Fuel Injectors

The hydraulically actuated electronic unit injector was technology developed by Caterpillar in the early 1990's. The most popular application of this technology was arguably the 7.3L and 6.0L Power Stroke diesel engines designed by International Navistar and used heavily in Ford 3/4 and 1 ton pickups. In fact, the International T444E and 7.3L Power Stroke are reportedly the first engine to employ the new technology on license from Caterpillar. A major benefit of the HEUI injection system was engine control, as the characteristics of injection events can be controlled independently of any mechanical system. The results were cleaner emissions and greater fuel economy potential.

How HEUI Injectors Work

A hydraulic electric unit injector applies the relationship between force, pressure, and area in order to create relatively high fuel injection pressures from pressurized engine oil. The process is similar in nature to the concept of leverage, where a force applied to a lever arm multiplies the torque applied at a fixed point - the longer the lever, the greater the torque multiplication. In hydraulics, however, the lever arm is replaced simply by a difference in area of a plunger, often referred to as an intensifier.

 HEUI intensification process

Visual representation of intensification process in a typical HEUI fuel injector

The figure above provides a visual representation of the intensification process used to increase output pressure for a given input pressure. In the case of an HEUI injector, the input pressure is the oil pressure in the high pressure oil system and the output pressure is the pressure of diesel fuel in the injector body. The plunger is simply a piston, commonly referred to as an intensifier in such hydraulic systems.

In a 1:1 system, where the plunger on the oil and fuel sides are of of equal area, the fuel pressure will equal the oil pressure. In a 2:1 system, where the plunger on the oil side is two times greater than the area of the plunger on the fuel side, the fuel pressure will be double that of the oil pressure. In a 7:1 system (which is the intensifier ratio on a typical 7.3L Power Stroke HEUI injector), the plunger on the oil side is seven times the area of the plunger on the fuel side - this translates into a fuel pressure that is seven times greater than the oil pressure.

The science behind the process is the relationship between force, pressure, and area. You may recall that pressure (P) is equal to the force (F) divided by the area (A) in which the force is applied to (P=F/A). Therefore, force must equal the pressure multiplied by the area in which the pressure is applied (F=P/A). With these formulas, we can express the process mathematically:

FP = Hydraulic force applied to intensifier
PO = Oil pressure
AO = Area of intensifier on the oil side of the injector
PF = Fuel pressure
AF = Area of intensifier on the fuel side of the injector


The pressure exerted on the plunger by the engine oil is equal to the pressure of the engine oil times the area of the plunger on the oil side.


The pressure of the fuel is equal to the force exerted by the plunger onto the fuel divided by the area of the plunger on the fuel side.

AO = 7AF or alternatively, AF = AO/7

The area of the plunger on the oil side is seven times greater than the area of the plunger on the fuel size (as is the case for the HEUI injector found on the 7.3L Power Stroke).

PF= 7(POAO)/AO which reduces to PF = 7PO

Substituting the equations in (1), (2), and (3) and reducing the equation yields that, in this instance, the pressure of the fuel is 7 times that of the pressure of the oil.

HEUI injector diagram

HEUI injector diagram

A typical injection event of an HEUI injector occurs as follows:

  1. The electro-magnet solenoid at the top of the injector is activated by the injector drive module (IDM) after an injection event is commanded by the powertrain control module (PCM).
  2. Once the solenoid is activated, the spring loaded poppet valve on the oil side of the system opens - high pressure oil fills the cavity and applies force to the intensifier plunger.
  3. The intensifier plunger applies force on fuel contained in the fuel cavity in the nozzle of the injector.
  4. Once fuel pressure is great enough to overcome the force of the spring on the nozzle valve, the nozzle valve lifts from its seat and fuel is atomized through the injector nozzle and is injected into the combustion chamber.
  5. Once the solenoid is deactivated, oil pressure drops, the poppet valve returns to the closed position, the nozzle valve returns to the closed position, and the cavity in the injector nozzle refills with diesel fuel.

High Pressure Oil System Overview

The fuel injector is just a small part of the injection system, which includes a high pressure oil pump (HPOP), an injection pressure regulator (IPR), the injector driver module (IDM), and various sensors feeding information to the powertrain control module (PCM).

High pressure oil pump - The HPOP is the heart of the HEUI injection system, creating and maintaining pressure in the high pressure oil circuit. The HPOP feeds highly pressurized engine oil to each injector and the pressure in this system dictates the operating fuel pressure. Oil pressure is dependent on engine load, not necessarily engine speed. For the 7.3L Power Stroke, operating pressures range from 500 to ~3,000 psi. For the 6.0L Power Stroke, operating pressures range from 500 to 3,600. This translates into maximum fuel pressures of 21,000 psi and 26,000 psi respectively. Both engines use a swash plate style hydraulic pump. Because of the extensive high pressure oil circuit, engines with HEUI systems typically have a relatively high engine oil capacity.

Injector Driver Module - While the PCM commands injection events, the IDM is what actually triggers the injector solenoids. The IDM is capable of delivering the voltage necessary to activate the injector solenoids. On the 6.0L Power Stroke, this device is called the fuel injection control module (FICM).

Injection Pressure Regulator - The IPR is an electronically controlled valve that regulates oil pressure in the high pressure oil circuit. The position of the valve changes with various parameters, including engine load and speed. When the system requires an increase in oil pressure, the regulator closes. Once the system reaches max pressure, the IPR will open, allowing excess pressure to bleed off.

Injection Control Pressure Sensor - The ICP sensor is a pressure sensor that relays actual hydraulic pressure of the high pressure oil circuit to the PCM. It is used to control the IPR, amongst other things.

Low Pressure Fuel Pump (lift pump) - The low pressure fuel pump, typically referred to as a lift pump in diesel applications, simply supplies fuel to each injector. Fuel is delivered to each injector at relatively low pressures (no greater than 100 psi).

Advantages of an HEUI Injection System

Keeping in mind that the HEUI injection system is long outdated and has been replaced by modern high pressure common rail technology, the HEUI injection system was advanced for the period it which it was developed. Many of the HEUI injectors advantages no longer exist when compared with modern injection systems. However, compared with the mechanical injection systems of the1980's and 1990's, the introduction of the HEUI injector offered the following benefits:

Greater Control of Injection Events - One of the primary benefits of the HEUI injection system is the unlimited control of injection events, something that had yet to materialize in the diesel sector. In a traditional mechanical injection system, injection timing and injector pulse width are determined by the settings of the injector pump and/or location of the camshaft. Although injection pumps are often adjustable to some extent, the characteristics of injection events are relatively fixed. HEUI systems, however, are electronically controlled and injection events and characteristics can be commanded dynamically based on various parameters. The HEUI injection system was developed in order to move away from camshaft controlled injectors, which exhibited no such flexibility.

Greater Injection Pressures, Improved Fuel Atomization - The HEUI injection systems on the 7.3L and 6.0L Power Stroke peak at 21,000 psi and 26,000 psi respectively. For comparison purposes, the International 6.9L and 7.3L IDI operate at injection pressures lower than 2,000 psi. Trucks equipped with the 5.9L 12v Cummins operated at fuel pressures less than 5,000 psi. Meanwhile, modern common rail engines see injection pressures near the 30,000 psi mark. The HEUI system, introduced in 1994, provided a significant improvement in fuel injection pressures.

The benefit of a higher injection pressure is greater fuel atomization and therefore a more efficiency combustion event. Atomization is the process by which liquid diesel fuel is vaporized through the injector nozzle, taking the form of minute droplets suspended in air. Complete atomization is highly desirable in any combustion process, as it promotes more complete and more efficient combustion.

Greater Fuel Economy, Lower Emissions - Greater atomization, injection pressures, and the flexibility to control injection events translates into greater fuel economy and lower emissions. Stringent emission regulations in the United States is a primary factory in the design (and redesign) of engines by U.S. manufacturers. The transition from the 7.3L IDI's mechanical injection system to the 7.3L Power Stroke's HEUI system proved highly beneficial in meeting fuel economy, performance, and emission requirements considering the engine would continue production through the 2003 model year.

Stiction in HEUI Fuel Injectors

Stiction occurs as result of oil break down and build up in the spool valve of a HEUI injector. The phenomenon can lead to rough cold starting, poor idle quality, and engine misfires. Stiction can be combated using a specially formulated oil additive and partially prevented by using a quality synthetic engine oil changed at or before OE recommended intervals.

Causes & Symptoms of Stiction

Stiction is caused primarily by the break down of engine oil due to intense pressure and heat. Under immense pressures, the molecules of the engine oil actually shear, modifying its chemical composition and fluid properties. As the oil deteriorates, a combination of carbon buildup, sludging, and varnishing occurs within the HEUI injector spool valve (oil side). As this phenomenon occurs it impairs the operation of the spool valve, contributing to inferior fuel atomization and irregular injection events. In addition, stiction increases wear between the spool valve plunger and bore. If neglected, irreversible damage may occur to the injector.

Common symptoms of stiction include:

• Hard to start, rough start, misfires after starting (engine sounds like it is not running on all 8 cylinders), especially when engine is cold.

• Visible smoke from tail pipe after starting, clears up once truck is warm.

• Engine misfiring when load is applied and engine is cold.

Curing Stiction Related Problems

There are many oil additives on the market that may help cure stiction related problems by removing buildup in the injector spool valve. We recommend Archoil AR9100 friction modifier , a widely popular oil additive that works well in eliminating and prevented stiction related problems, including injector misfires. If an injector has incurred physical damage or is malfunctioning as a result of excessive wear, don't expect an oil additive to be a cure-all. The most beneficial course of action is prevention - start an oil additive regiment to prevent problems before they occur.

Stiction Prevention

Stiction is extremely common for the 6.0L Power Stroke and is, to some extent, preventable with the use of an oil additive specially formulated to combat it. However, proper maintenance remains imperative in reducing its severity. Using synthetic engine oil, which is more resilient to thermal, chemical, and mechanical break down can greatly reduce coking/buildup in the injector. In addition, be mindful that the OEM maintenance schedule reduces oil change intervals in trucks that operate in "severe" duty conditions, which includes frequent towing. Follow the OE recommendations and remember that the harder the truck is worked, the more frequently it should be serviced.