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2004.5-2005 Duramax 6.6 LLY

General Motors introduced the LLY Duramax diesel engine in 2004 as a response to stricter emissions requirements set by the federal government and California. The LLY replaced the LB7 Duramax and was in production until 2006 when it was replaced by the LBZ Duramax. Although the LLY faced additional emissions challenges, it was a powerful and capable engine for its time.

The LLY produced 310 horsepower and an impressive 605 lb/ft of torque. It was paired with a 5-speed Allison transmission in 2004 and 2005, and the Allison transmission was upgraded to a 6-speed in 2006. The LLY was first introduced in the Chevy Silverado and GMC Sierra 2500HD/3500HD pickups halfway through the 2004 model year. The carryover LB7 and the new LLY engines were both available in the same year, leading diesel enthusiasts to refer to the LLY as the 2004.5 model year.

The LLY utilized some of the components from the previous LB7 Duramax but included new components that increased performance. The short block featuring a cast-iron block with aluminum heads was carried over from the LB7. The high-pressure common-rail fuel system with a Bosch CP3 injection pump, which debuted in 2001, was also still in use. However, new externally serviceable fuel injectors were added to avoid the injector issues that plagued the LB7.

The LLY was a 6.6L turbocharged engine with a 32-valve design, high-pressure common-rail direct injection, and aluminum cylinder heads. It was GM's first attempt to meet emissions requirements for their diesel trucks. To achieve this, they installed a newly developed Garrett turbocharger with a variable geometry vane system and an EGR Valve.

To address the injector problems that arose with the LB7, GM designed the LLY with valve covers that allowed easy access to the injectors without having to remove the valve covers, which saved on labor costs if injector replacement became necessary.

The LLY Duramax was a stepping stone for GM as they began to encounter stricter emissions requirements. The LLY was a powerful and capable engine, and the addition of the new components increased its performance.