Harness Rub Through
A common problem on 2004.5- 2006 Duramax LLY engines is what's typically known as harness rub. The injector wiring harness is zip-tied to a metal flange next to the alternator. The problem is that the harness loom stops too short, leaving just a thin layer of protective material between the metal flange and the wires inside the harness. Over time, vibrations cause the metal to rub through the thin layer and into the wires' insulation, eventually exposing the wires themselves. The best preventative fix is to cut the zip tie and cover the harness with more wire loom or a chunk of rubber hose-basically anything that can act as a guard between metal and wire-and zip tie it securely in place against the metal flange.
Air Intake Heater
The 1999.5-2003 7.3L Power Stroke came equipped with a factory Air Intake Heater (AIH) that can be found in the intake tract. The AIH is modulated by the pcm via a relay that is located next to the glow plug relay. This intake heater protrudes directly into the air stream in the cold side of the intake manifold and blocks approximately 20 percent of the intake path.
While the design and use of the AIH may have good reasoning behind it, which is to heat incoming air to alleviate white and gray smoke while the engine is cold. The factory programmed pretty specific circumstances into the PCM before it will power up the relay and engage the AIH to operate. Meaning, the AIH is rarely used, and on most vehicles, especially those in warmer climates, it is not needed.
It's a pretty common practice to remove the AIH completely from the truck, freeing up some of the space in the intake tract, allowing for better airflow. If it's not being used, why obstruct and inhibit airflow to the motor, right?
Removal of the AIH is simple, and will have no negative effects on operation of your engine. First, you'll need to disconnect the wiring that runs from the AIH to the relay. Start by removing the black ground wire, and then disconnect the red wire from the heater, following that wire to the relay/solenoid. Loosen the nut holding the red wire on and remove the red wire. Be sure to leave the blue wire connected to the relay; it needs to stay. You can then remove the AIH from the intake manifold. Once it's out you'll see how much of an obstruction it actually is. To plug the open port in the intake manifold, run to the local parts house and purchase a standard 5.9L Cummins engine oil pan drain plug. This oil pan plug is the same thread pitch and will have a gasket on it to create a tight seal. Simply install it where the AIH once was and snug it down.
Exhaust brakes put an unusual amount of compression between the exhaust side of the engine and the exhaust brake itself. When the brake is engaged, the turbo, manifold and gaskets are subjected to much higher pressures than they normally experience without an exhaust brake.
Because of this, it's important to keep an eye for exhaust leaks in this area. Watch for soot marks around joints, gaskets and fittings. The exhaust manifold-to-turbo gasket pictured below has been worn away from the inside out as a result of these higher pressures. It was obvious when this leak appeared because when the brake was activated during engine warm up, the leak produced a high-pitched whistle as the back-pressure escaped.