Bosch Common-Rail Injection
The Duramax engine employs a common-rail fuel system. Each bank of cylinders is supplied with fuel from its own rail, just as a multi-port gasoline engine would be.
The fuel injection pump, known as a CP3 pump, is located in the front of the engine "V" and is gear-driven from the camshaft and controlled electrically by the ECM through a fuel rail pressure regulator.
The injection pump has a fuel supply pump that is located at the rear of the unit, along with a high-pressure pump. There is no external lift pump on GM pickup trucks.
The supply pump moves fuel from the fuel tank to the inlet port of the high-pressure pump. The fuel pressure regulator varies the amount of fuel from the supply pump to the high-pressure pump in order to control the output pressure. The fuel injection pump generates a peak fuel pressure of approximately 26,000 psi. Typical pump pressure at idle is 5,000 to 6,000 psi.
The manifold absolute pressure (MAP) sensor is used to determine engine load and boost pressure coming into the engine. | 
The camshaft position sensor is required to identify which cylinder to fire, and in which sequence. | 
Beginning with the '04 1/2 LLY Duramax engine, the Garrett turbocharger uses an electronically controlled hydraulic actuator to control the vane position. |
Fuel Injection Control Module (FICM)
The timing of the fuel injection is controlled by the ECM through the fuel injection control module (FICM), which is a driver box (a driver is an electronic relay with no moving parts). The amount of fuel injected is determined by the fuel injector "On" time. Current flow to the injectors is controlled by the FICM. The ECM communicates with the FICM through a CAN (Controller Area Network) serial data link.
The engine uses two types of injection pulses for fuel delivery. Pilot injection is a small amount of fuel introduced to promote combustion efficiency and reduce combustion noise. A normal (main) injection immediately follows the pilot injection. Pilot injection strategy is not used at engine speeds above 2,500 rpm.
The LB7 Duramax was the first light-duty diesel truck engine that used a mass airflow sensor (MAF) to measure the amount of air coming into the engine. It is located in the ducting that runs from the factory airbox to the turbocharger inlet.
The FICM is mounted on the passenger-side valve cover and is cooled with diesel fuel in order to reduce the heat produced by the electronic drivers. The electrical power required for fuel injector operation is 93 volts DC. As much as 20 amps of current is required to open a fuel injector, while 12 to 15 amps are needed to hold the injector open against fuel pressure.
The '04 1/2 LLY version had a number of changes. The FICM was a new design that allowed "top-to-bottom" fuel flow through the base of the unit for cooling purposes.
Injectors
The fuel injectors used for '041/2 to '05 LLY Duramax engines had a six-hole spray pattern from the tip while the '06 version of the LLY employed a seven-hole design that also featured a double-guided and modified case that was 25 percent harder. The step bore improved installation with less chance of damaging the pintle.
The crankshaft position sensor is used to determine engine rpm. It's used in conjunction with the camshaft position sensor to determine which injectors to fire.
The later LLY, as well as newer versions of the Duramax, used a new injector identified as an IQA, or injector quantity adjustment. It is also referred to as injector flow rate programming (IFRP), and involves values assigned to each injector after flow testing at the production plant. During the manufacturing process, each injector is tested and its flow rate is measured at several duty cycles. These measurements are recorded as the injector flow rate value. This data is then recorded on the bar code label. It is laser etched as a hex number on the body of the injector before it is shipped to the Duramax engine plant in Moraine, Ohio.
The IQA values for each cylinder are written in the GPCM memory at the engine assembly plant. During final vehicle assembly (at another location), the values and cylinder position information from the GPCM are copied and written to the ECM memory.
The vehicle leaves the assembly plant with two identical copies of the stored IQA values, one in the GPCM and the other in the ECM.
The IQA values are used by the ECM to fine-tune the fuel delivery to each cylinder under all engine operating conditions.