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Duramax History, Lesson 2: LLY

Midway through the 2004 model year, GM released the second-generation Duramax, RPO code LLY. Although much of the LLY’s internals match what you’ll find in the LB7, this version of the 6.6L V8 brought an improved injector design, variable geometry turbocharging and cleaner emissions to the table. On early versions of the LLY (’04.5-’05.5 model year GM trucks), horsepower climbed by 10 to 310hp while peak torque remained at 520 lb-ft—but became available 200 rpm sooner in the power band (1,600 rpm vs. 1,800 rpm on the LB7). Later model LLY mills (‘06), which arguably had more in common with the LBZ that followed the LLY, packed 605 lb-ft of torque.

However, despite its improvements and added technology over the LB7 the LLY wasn’t without its own shortcomings. Issues with overheating, brought about from various different components, proved to be its biggest drawback, often contributing to higher operating temps and sometimes even blown head gaskets. While previous injector issues were laid to rest with the LLY, the common-rail injection system still had a few minor quirks, along with the injection system still being void of a factory lift pump. But even though it tends to be viewed as the least desirable in the Duramax lineage, the LLY was a very solid engine. For all the dirt on this briefly-produced and often-overlooked oil-burner, keep scrolling.

Make sure to check back in for Part Three, as we’ll be exploring one of the most coveted Duramax engines ever built: the LBZ.

LLY Hard Facts

Production 2004.5 to early 2006 Heads Cast-aluminum with six 14mm diameter head bolts per cylinder (with sharing)
Design 90-degree V8 Valvetrain OHV, four valves per cylinder, single cam
Bore 4.06 inches Injection System Bosch high-pressure common-rail, direct injection
Stroke 3.90 inches Injectors Bosch solenoid (outside valve cover)
Displacement 6.6L (403 ci) Injection pump Bosch CP3
Compression Ratio 17.5:1 Turbocharger Garrett GT3788VA
Block Deep Skirt, cast Iron (gray iron alloy) Intercooler Air-to-air
Rods Forged-steel, fractured (cracked) cap Horsepower 310hp @ 3000 rpm
Pistons Cast-aluminum Torque 520 lb-ft @ 1600 rpm (early '06: 605 lb-ft @ 1600 rpm)

Boiling Over


The commanding issue most LLY owners worry about is this engine’s tendency to overheat, especially when towing. In reality, not all trucks overheat, but it’s a common enough problem that it gets a considerable amount of negative attention. Several factors contribute to this engine inherently running warmer than its LB7 predecessor (or the engines that followed it), one of which is the fact that LLY-powered GMs came equipped with the smallest radiator ever offered in front of a Duramax (yet had an additional component to cool it in the form of the EGR cooler). While the employment of a radiator with less surface area and lower coolant capacity certainly didn’t help the LLY, the overheating hysteria is often exacerbated by owners who experience fan clutch failure, water pump failure or a debris-packed cooling stack blocking airflow, but blame their 240-degree water temp on the radiator.

Turbo Inlet Manifold


Another key contributor to the LLY’s running warm is the factory turbo inlet manifold, or turbo mouthpiece as some folks know it. Highly restrictive, the turbo inlet manifold creates a choke-point right before boost enters the compressor side of the turbocharger. This makes the turbo work harder to produce boost, which in turn leads to hotter intake air and exhaust gas temperatures, and often hampers spool up, fuel economy and overall power. Replacing this piece with a freer-flowing aftermarket turbo mouthpiece is one of the most affordable and effective ways to help your LLY run cooler.

More Airflow Restriction


Although it isn’t near the choke-point the turbo inlet manifold is, the factory two-piece intake Y-bridge is also known as a considerable restriction on all Duramax engines. Larger diameter aftermarket Y-bridges (a WCFab kit is pictured above) abound in the diesel aftermarket, which not only improve airflow into the heads, lower intake and exhaust gas temps, but also rule out the two-piece component’s propensity to blow apart under high boost pressure.

Variable Geometry Turbo


Meeting emissions standards directly correlates to good drivability—and a quicker-spooling turbocharger means less particulate matter pollutants. To make the engine as responsive as possible throughout the entire rpm range, variable geometry turbo technology debuted on the LLY in the form of the Garrett GT3788VA VVT. With precise electric control of the movable vanes that direct exhaust flow across the turbine wheel, optimum performance is achieved at virtually any engine speed. The result is superb drivability in any situation and the faster spoolup provided by the GT3788VA VVT provided for peak torque to check in sooner (at just 1,600 rpm).

Largest Turbo


One benefit for horsepower junkies is that the LLY made use of the largest turbo ever offered on any Duramax engine. Not only was its 62mm, 11-blade compressor wheel inducer the biggest, but the height of its exhaust vanes were the tallest at 15mm (vs. 11.2mm on the LML engine available from ’11-‘16). This translates into the LLY possessing the highest flowing factory turbocharger. With sound aftermarket tuning, it’s a cinch to clear 500rwhp with an LLY-powered Chevy or GMC HD.

Performance-Friendly Turbo Contributes to Blown Head Gaskets


Unfortunately, being the biggest doesn’t always mean it’s the best—especially in the case of the LLY. With a bigger compressor wheel to turn yet more restriction in front of it (in the form of the aforementioned turbo inlet manifold), the variable vanes in the turbo that are essentially used to drive the compressor wheel have to work harder. This means more drive pressure and heat is produced, which not only leads to higher coolant temps, but over time takes its toll on the LLY’s head gaskets.

Emissions System


Although exhaust gas recirculation (EGR) began to infiltrate the Duramax on California model LB7s, the LLY was 50-state legal right out of the box. Its EGR system consisted of an EGR valve to introduce exhaust gases back into the intake stream and an EGR cooler in the valley. Downwind of the turbocharger, a catalytic converter was employed as well. Neither the EGR valve, cooler or cat on the LLYs are notorious for failure (such as the 6.0L Power Stroke from the same era), but it does happen on higher mileage engines.

Same Rods, Same Tolerance for Pain


The same forged-steel, cracked cap connecting rods found in the LB7 were repurposed in the LLY. Of the first five generations of the Duramax (LB7, LLY, LBZ, LMM and LML), the LB7/LLY rod is the second lightest ever used. At 1,155 grams apiece, only the rods used in the LML (’11-‘16) were lighter. However, just like the LB7, the rods are on borrowed time at any point beyond 600rwhp. A relatively high compression ratio when compared to later Duramax mills (17.5:1 vs. 16.8:1) and its use of a quick-lighting, low-rpm torque-producing variable geometry turbo makes the LLY rods slightly more susceptible to bending than the LB7’s.

Redesigned Injectors           


Seeking to solve the widespread and burdensome injector issues that plagued the LB7—the one major blemish on the original Duramax’s otherwise stellar platform—GM obtained newly designed, externally serviceable injectors from Bosch and redesigned its cylinder heads to accommodate them. The results were exactly what GM wanted and needed: no more leaking injectors, cracked injector bodies or rampant warranty claims. While fuel quality issues, poor fuel lubrication and air infiltrating the fuel system (thanks to the LLY’s lack of a factory lift pump) can lead to injector issues as early as 125,000 miles, it’s a far cry from the frequent and catastrophic injector issues that haunted the LB7. Most well-maintained LLY injectors last between 150,000 to 200,000 miles.

Injector Harness Chafing


High mileage and age often bring about inevitable electrical problems on any modern diesel. After years of use and hundreds of thousands of miles, the LLY’s injector harnesses are known to wear through due to vibration and throw injector output circuit DTCs. Short-to-ground scenarios are common when this happens, but it can also result from the injector output circuit seeing high resistance. If the problem area can be tracked down, it can usually be repaired without replacing the entire injector harness.

‘Ol Reliable


For good reason, the injectors in the LLY are completely different from the LB7, but GM decided to leave the CP3 virtually unchanged between the two engines. After all, Bosch designed this injection pump for engines much larger than the Duramax and that were expected to last well beyond 500,000 miles. Similar to the LB7 pump, a suction valve is integrated to compensate for the absence of a lift-pump. Also like the LB7 pump, a fuel pressure regulator replacement is in store once in a while, but the CP3 itself is rock-solid.



This applies almost exclusively to enthusiasts who’ve tapped into the hidden potential of the LLY via aftermarket tuning. The factory fuel pressure relief valve (FPRV) is a spring-loaded plug located at the back of the driver side fuel rail that’s designed to protect the rail in the event of an over-pressure situation. But once the FPRV spring has allowed fuel to be discharged, it becomes weaker and weaker and the engine’s peak obtainable rail pressure drops lower and lower (i.e. your truck slowly makes less and less power). The issue can easily be solved by installing an aftermarket race valve or by shimming the factory FPRV spring to raise its relief pressure back to (or above) stock pressure. A $10 shim kit from Merchant Automotive is shown above.

Missed the first lesson? See the history of the LB7.

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