When the Duramax was released in 2001, GM finally had a diesel that could compete with those offered by Dodge and Ford. While the earlier motors had some teething problems, by 2006 they had mostly been ironed out with the introduction of the LBZ. This latest version featured some upgraded internals and received a boost in power, now putting out 360 horsepower and 650 ft/lbs. of torque. The LBZ was an instant hit and more powerful than anything GM’s competitors had to offer at the time.

While the LBZ has been out of production for 15 years now, its reputation as a “hot rod” and tuner’s engine endures to the present day. In fact, it is considered the most desirable of all the Duramax engines due to its reliability, simplicity, and the fact it marked the end of the “pre-emissions” era.

Origin Story

Born in 2001 through a partnership between GM and Isuzu, the 6.6L Duramax V8 was a vast improvement over the Detroit Diesel it replaced. Over the last 20 years, it has gone through several revisions with the latest version, the L5P, powering many GM light and medium-duty trucks.

• LB7 (2001 to 2004)

Introduced in 2001 and putting down 300 horsepower and 520 ft/lbs. of torque (good numbers for the time) GM was now in the lead. Not only was the new Duramax powerful, it was also the first diesel engine to use a common rail direct fuel injection. However, it was not without its problems, namely faulty fuel injectors and a weak head gasket.

• LLY (mid-2004 to 2006)

The first update came in 2004 with the most notable improvement being a larger variable geometry turbo (the largest installed on a Duramax to date) that increased torque from 520 to 605 ft/lbs. by 2006. By this time, the issues with the fuel injectors and common rail had been resolved although the LLY was prone to blowing head gaskets (mainly due to the larger turbo). Additionally, the cooling system was not up to the task and overheating problems were frequently reported when towing.

• LBZ (2006 to mid-2007)

By the time the LBZ was released in 2006, the aforementioned problems had been worked out. This was also the first revision of Duramax, as the new LBZ had beefier internals and an upgraded fuel system. Even though the turbo was slightly downsized, horsepower and torque were increased to 360 and 650 respectively. In addition, a more capable six-speed Allison 1000 transmission was introduced.

However, the LBZ was short-lived due to tightening emissions control standards and was replaced by the LMM in 2007.

• LMM and later (mid-2007 to present)

The LMM retained its predecessor’s engine upgrades although the introduction of new emissions control equipment such as Diesel Particulate Filters created new reliability problems that took a few years to resolve. By 2011, they were mostly ironed out with the introduction of the LML which had upgraded internals along with a sizeable increase in power. However, this often caused the common pump to fail prematurely, a costly repair.

The L5P, the current version, resolved this problem, and power was boosted again from 397 horsepower and 765 ft/lbs. of torque to 445 and 910 respectively.

Despite having been surpassed in the power department for a decade now, the LBZ remains popular due to its improvements over the LB7/LLY along with its simplicity, reliability, and ease of tuning when compared to the LMM and later engines.

LBZ Engine Specifications & Tech Dive

Block

As mentioned above, the LBZ was the first major refresh of the Duramax, and the block was strengthened to handle the extra power. Among the improvements were more webbing in the areas of the main bearings and taller main bearing caps with a 4mm deeper bore for the cap bolts. While this was achieved by slightly decreasing the size of the oil feed holes, it had no detrimental effects in keeping the critical parts of the engine lubricated.

Suffice to say, there are other things that will give out well before you need to worry about the block.

Rotating Assembly & Top End

The top end was largely unchanged from previous designs although the LBZ received some beefier connecting rods with more meat on the lower (crank side) end. While heavier, they were less prone to bending and breaking as opposed to those used in the LB7/LLY.

However, a new problem emerged with the LBZ in the form of cracked pistons that were not present in the earlier engines, and it would remain a recurring theme in its successor, the LMM.

It should be noted this problem is mostly limited to high-horsepower applications (650+), well beyond the levels achieved by simple tune and bolt-on modifications.

Turbo & Fuel System

To handle the extra power, the Bosch CP3 common rail was modified to provide extra fuel pressure, increasing it to 26,000 psi (up from 23,000 psi) along with an upgraded fuel rail and bigger injectors.

The turbo was slightly downsized, albeit a larger inlet manifold was installed which had been a bottleneck on the LLY. This not only improved the turbo’s efficiency but reduced lag as well. Despite that, clogged turbo vanes were a frequent occurrence due to carbon buildup or corrosion. Granted it was more of a nuisance and could usually be cleared with some spirited driving, or in the worst case, removing the turbo and cleaning it.

LBZ Reliability

By the time the LBZ debuted in 2006, many of the teething problems with the earlier Duramax engines had been worked out. Additionally, it was not burdened by the reliability issues that plagued the later LMM/LML. In fact, the LBZ Duramax is generally regarded as the most reliable version to date.

With that said, (and now being 15 years old) most of the problems will be due to age, how well it was maintained, and mileage. However, if you are in the market for an LBZ Duramax, here are some things you should keep in mind.

Water Pump Failure

The LBZ and later engines were prone to water pump failures, usually resulting from the plastic impeller breaking off inside the pump. The tell-tale signs of this will be an overheating engine and a coolant leak around the water pump. If you are in the market for an LBZ and the pump has 80,000 or more miles on it, you may want to consider replacing it.

Fortunately, it is not a costly repair and there are aftermarket water pumps that resolve this issue and do not cost more than a few hundred dollars. At the same time, it can be done by a home mechanic with basic hand tools.

Leaking Transmission Lines

While the Allison 1000 is a stout transmission, the coolant lines are notorious for leaking due to a poor crimp design and this was a known problem even in 2006. Chances are if you find transmission fluid on the ground, replacing the coolant lines is in order. Generally, it occurs when the vehicle is cold and is usually nothing more than an occasional drip.

Fortunately, it is not a deal-breaker and like the water pump, it can be fixed for a few hundred dollars by installing some upgraded transmission cooling lines.

Cracked Pistons

It’s no secret the LBZ can handle some serious power although the pistons are the weakest link. While there is some debate as to the actual cause, whether it be quality control issues or the wrist pin bushings, it was not resolved until 2011. However, reaching the power levels where this tends to happen (usually over 650 horsepower) requires extensive mods like bigger turbos and fuel system mods that cost well over $10,000 so for most people, this will not be an issue.

Suffice to say, replacing a cracked piston is an expensive repair as it necessitates an engine teardown, easily costing thousands of dollars.

Trust The LBZ Duramax Experts

When it comes down to it, there isn’t a group of diesel experts in the country that knows more about the Duramax and Allison transmission than Merchant Automotive. With thousands of hours of collective experience, our products are carefully engineered to solve common problems with every Duramax generation, including the LBZ, while also coming with the customer service that our valued clients deserve.

Check out our huge selection of Duramax / Allison parts and accessories for your truck today!