The LLY (2004.5–2005), LBZ (2006–2007), and LMM (2007.5–2010) Duramax engines share the same core high-pressure fuel architecture: the durable Bosch CP3 pump (building over 20,000 psi) feeding solenoid-style common-rail injectors. The CP3 has a strong reliability reputation, so most running problems on these trucks trace to the injectors or fuel supply rather than the pump.

Common symptoms

• Rough idle or a dead miss on one cylinder.
• Hard start and white smoke, especially when cold.
• Knock or rattle under light load.
• Fuel in the oil (rising level) from an injector leaking into a cylinder.
• Reduced power and fuel economy.

Note: the LLY is also known for overheating when worked hard, which can be mistaken for a fueling issue — confirm cooling-system health when diagnosing.

How to diagnose it

1. Pull injector balance rates with a scan tool to flag the weak cylinder(s).
2. Perform a return-flow (spill) test — excessive return from one injector indicates internal wear.
3. Check fuel-rail pressure against spec to separate a pump/supply issue from an injector issue.
4. Check oil level/quality for fuel dilution.
5. Rule out the basics — fuel filter, water in fuel, and battery/cranking health.

Repair notes

Injectors on these engines are frequently replaced as a set when several are worn. The CP3 itself is robust and often outlasts the injectors, so confirm which component is actually at fault before ordering.

Before you order: LLY, LBZ, and LMM injectors and pumps are application- and year-specific. Verify the exact part by OEM cross-reference for your truck before purchasing.

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