The 6.6L Duramax has been in production since 2001 and has proved, on a large scale, to be a relatively reliable and durable platform. While various versions of the engine have surfaced over the years, all generations of the Duramax are based on the same foundation. Each generation is more-or-less an evolution of the previous model, with improvements and modifications made as necessary to maintain a competitive edge in the marketplace and meet progressing emissions standards. However, like any engine (or any complex mechanism for that matter), the Duramax engine and Allison 1000 transmission have some common problems inherent to their design. Some of these issues can be prevented via modifications while others may be avoided by simply understanding the root cause(s).
Fuel Starvation and/or Air in the Fuel Lines
The Duramax diesel is susceptible to fuel starvation and getting air in the fuel lines. This can be contributed to the fuel filter housing design and the fact that the Duramax does not use a lift pump. Rather, the high pressure fuel pump (injection pump) is responsible for vacuuming fuel from the tank. It is quite common for the fuel filter housing to develop a small crack or let air seep in due to a bad housing o-ring or water-in-fuel sensor o-ring. It can also be relatively difficult to prime the fuel system once air has been introduced into the system, such as when the fuel filter is changed. The 2001 to 2004 LB7 fuel filter housings seem to be the most prone to problems.
Water Pump Failure
Water pump failures for Duramax engine are somewhat common. It is not unlikely that a water pump will need to be replaced in the 80,000 - 100,000 mile range. This seems to be inherent of the factory water pump design.
Overheating issues with the Duramax diesel seems to be a hit-or-miss problem; some owners experience it, while others never will. Regardless, it's more common with the Duramax than other engines. Overheating typically occurs while towing in the Summer months, and is most prevalent in 2005 or earlier models (2006+ models received a larger radiator and fan). Overheating often occurs as a result of a fan clutch failure, preventing the engine fan from providing supplemental airflow as necessary to keep the engine from overheating. There's some speculation that a dirty/clogged radiator also contributes to the problem, as the grime that builds up on the radiator over time reduces its effectiveness as a heat exchanger. A water pump failure (which is relatively common) will also likely result in overheating.
Premature injector failure was a critical problem for the 2001 to 2004 model year Duramax LB7. GM corrected the problem with an updated injector design and even extended the warranty on the new injector design to 7 years/200,000 miles. If you are buying a used truck, be warned that not all engines have been retrofitted with the new injector design. The design flaws of the original LB7 injectors have not been characteristic of later injector designs. However, Duramax injectors (in general) seem to be relatively sensitive to contamination and may fail prematurely if proper maintenance is not performed at regular intervals.
Injector Harness Chafing
It is extremely common for the injector wiring harness to chafe over time. This can cause a number of issues if the wires become exposed, including a no-start or rough running, lack of power situation. The associated trouble codes will usually reveal which injector(s) are affected and will relate to an "open circuit" condition. Example: "Injector circuit open, cylinder 7". To relieve this problem, the wiring harness (or affected portion of the harness) will need to be replaced/repaired. Often, you can verify the problem by wiggling the wires around the injector. If the problem goes away (even momentarily), you've identified the affected zone.
Glow Plug Failure/Concerns
Glow plug failure was a concern on the 2006 model year LBZ and LLY Duramax models, but the problem should have been long corrected for owners of these years. The concern was that the glow plug module would over cycle the glow plugs, creating a scenario in which the tip of the glow plug could actually deform and/or break off in the engine. In such an instance, catastrophic engine damage could (and likely would) occur. GM identified the problem and has reprogrammed the glow plug module to eliminate this risk.
Aluminum Cylinder Heads
The truth of the matter is that there are no concerns regarding the aluminum cylinder heads on any generation of the 6.6L Duramax. With over 1,000,000 Duramax engines on the road today, the aluminum cylinder head design has yet to be identified (with any credibility) as an inherent problem for Duramax owners. The concept of aluminum cylinder heads on a diesel engine, which is subjected to relatively high cylinder pressures, is more worrisome than the reality. The head bolt design has proved sufficient in preventing the cylinder heads from lifting under the conditions they are subjected to (at stock power levels). In fact, the Duramax is no longer the only engine featuring aluminum cylinder heads; Ford's 6.7L Power Stroke also utilizes the lighter material and their design has proven equally reliable. When the Duramax was first introduced, many were skeptical about the aluminum cylinder heads, but there is no need to worry.
PCV Design - Turbocharger Oil Ingestion
The Duramax PCV design vents the crankcase pressure into the intake. As a byproduct of the process, engine oil is introduced into the turbocharger. Over time, this engine oil coats the inside of the intercooler and intercooler tubing. The primary concern is that large quantities of oil settle in the intercooler boots, causing them to rapidly degrade. There are several aftermarket products on the market that reroute the PCV line so that it vents into the atmosphere rather than into the intake, and a number of "DIY" methods of remedying the problem.
Allison 1000 Transmission Limp Mode
The Allison 1000's limp mode feature is a safeguard against catastrophic transmission failure. In reality, it's less than ideal for many situations. When the Allison 1000 transmission senses an abnormal amount of slip in the transmission, it enters limp mode. While in limp mode, the transmission will lock in 3rd gear, the torque converter will remain unlocked, and the transmission will not be able to operate in reverse. It's great to have such a fail safe, but in many instances the limp mode is activated when it's not entirely necessary. It usually occurs under heavy load (such as towing, down shifting to pass, etc), and is extremely common in tuned trucks (even a mild performance increase can cause enough slippage to trigger limp mode). There is concern that the transmission could slip in high gear and be forced into 3rd gear suddenly, causing a spike in engine speed. To take the transmission out of limp mode, clear the engine trouble codes using a scan tool. Some owners have also suggested that turning the engine off and cycling the key to the on position several times will take the transmission out of limp mode. If your transmission experienced limp mode, it's likely on the verge of needing a rebuild in the near future and should not be ignored.
2011 LML DEF/SCR System Woes
The Duramax LML was introduced for the 2011 model year featuring the most sophisticated emissions system yet. The latest additions included the selective catalyst reduction (SCR) system, which requires a constant supply of diesel exhaust fluid (DEF) to inhibit the reaction that converts nitrous oxides into more impregnable compounds. The system relies on various sensors plumbed into the exhaust system in order to accurately control the injection of DEF into the catalyst. Not particularly surprising, the new system proved littered with problems, most of which would be ironed out by the production of the 2012 model year engines. By far and large, the most common problem was the reoccurring failure of one of the systems two NOx sensors. Additional problems included DEF level sensors, DEF pumps, and DEF tank heaters (which often allowed DEF to freeze in extremely cold environments). Owners of 2011 model year trucks may or may not be covered by an extended warranty on NOx sensors, as the widespread frequency of problems prompted GM to increase the warranty period for these parts for what they consider "affected trucks".