The fuel injection system was called Hydraulic Electronic Unit Injection system or HEUI for short. This system employed two different systems to inject the fuel at incredible pressures (up to 21,000 PSI). The hydraulic system is a high pressure oil system that feeds the fuel injectors up to 3,000 PSI of oil pressure. When the Injector Driver Module (IDM) tells the injector to fire, it opens a poppet valve. The poppet valve allows the pressurized oil from the High Pressure Oil Pump (HPOP) to enter the injector oil cavity. At the bottom of the cavity is an intensifier piston that the oil presses down on. On the other side of the piston is the fuel. So when the oil starts pushing down on the intensifier piston, the piston pressurizes the fuel by 7 times. Once the fuel pressure has reached approximately 2,700 PSI, it overcomes the injector pintle and the fuel starts to be injected into the cylinder.
The combination allowed the new 7.3L DIT's to be extremely powerful and relatively clean. The 21,000 PSI of fuel pressure is the main reason for this. Today, EPA-certified diesel engines inject the fuel between 25,000 and 32,000 PSI, but in the early 1990's only the 7.3L DIT's light-duty truck engines were firing at this pressure range.
In 1999 Ford introduced a new body style truck: the Super Duty. The Super Duty still featured the 7.3L DIT, but with a few revisions. Ford added a larger turbocharger, an intercooler, and they made some electronic changes. As emissions laws started to change and the horsepower and torque wars started to heat up, Ford decided it was time to redesign the engine again.
In late 2003, Ford introduced the next generation of Powerstroke. This engine was quite small at 6.0 Liters of displacement, but there were many improvements that made it more powerful. These engines had a better fuel consumption rating (better MPG), higher horsepower and torque ratings and the emissions were reduced. These engines were higher reving, more powerful and quieter on the road. One of the main reasons for them being quieter is they revised their HEUI system.