About HEUI Injectors
The concept of HEUI injectors, or hydraulic electric unit injectors, pre-dates the common rail and was developed as a more efficient alternative to mechanical injector designs of the period. Though the technology is now obsolete, it was considered the advanced design for many years. The HEUI concept relies on the engineering principles surrounding pressure, and uses engine oil as a working fluid to pressurize fuel. The system is relatively simple - a high pressure pump pressurizes engine oil in each individual injector where the pressure of the oil acts upon the fuel supply in the body of the injector by means of a piston or plunger, creating a fuel pressure higher than that of the oil pressure. For Ford pickups, the HEUI was employed on the 7.3L and 6.0L Power Stroke. Depending on the engine, oil pressures can max out in the 3,000 to 4,000 psi range, producing fuel pressure as high as 26,000 psi prior to an injection event.
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's AR9100 friction modifier (it can be purchased here, 16 oz is needed per oil change) - it's widely popular, works well in eliminating stiction related misfires, and is reasonably priced. There are alternatives on the market and we encourage owners to select a product based on their own research; opinions are subjective. 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 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.