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Aug 21, 2023

Duramax History, Lesson 4: LMM

Increased emission regulations began to alter the diesel landscape significantly

Increased emission regulations began to alter the diesel landscape significantly in 2007. That year, Ford, Dodge and GM all released engines designed to meet the new particulate matter (PM) and nitrogen oxide (NOx) limits mandated by the Environmental Protection Agency. For GM, an updated version of the 6.6L Duramax, RPO code LMM, was all that was in store, whereas Ford and Dodge unveiled completely new power plants (the 6.4L Power Stroke and the 6.7L Cummins, respectively). Basically an LBZ saddled with an intricate exhaust after treatment system, the LMM remained architecturally similar to the Duramax mills that preceded it. Despite the added complexities of the new emissions control system, GM was still able to increase power output and remain a step ahead of Ford and Dodge.

The LMM Duramax brought 365hp and 660 lb-ft of torque to the table along with its emissions equipment, including exhaust gas recirculation, a diesel oxidation catalyst and a diesel particulate filter. Its bore of 4.06 in and stroke of 3.9 in also created a displacement of 405 ci.

As is the case with most modern diesel engines, emissions-control devices account for the majority of premature failures. The LMM is no different, with its labyrinthine-like exhaust after treatment also contributing to poor fuel economy, diluted engine oil and reduced long-term engine longevity. In the aftermarket, cracked pistons remain common thanks to the LMM's use of the same pistons employed in the LBZ. But outside of those shortcomings, the LMM came with several upsides. 1) It received cast-aluminum heads superior to any other Duramax produced, 2) it was equipped with revised injectors which provided both added power and more complete combustion and 3) it came bolted to the all-new GMT900 platform and was wrapped in a fresh body style.

Don't forget to tune in for Part 5, where we spotlight the LML—the Duramax that brought more robust internals, a higher pressure injection system and additional emissions equipment to the table.

It was during the LMM's tenure that GM produced is 1 millionth Duramax. In April of 2007, engine number 1,000,000 was put on display at DMAX Ltd. headquarters upon final assembly. Fun fact: Every completed 6.6L Duramax undergoes an 8-minute hot-test on an engine dyno to verify its performance and build quality before it's allowed to leave the Moraine, Ohio production plant.

In order to lower particulate matter emissions by 90 percent, the LMM came equipped with a diesel particulate filter (DPF) located down stream from the diesel oxidation catalyst (DOC). Designed to trap harmful pollutants that aren't completely burned off in the combustion process, the DPF eventually accumulates enough soot to be periodically burned off through a process called regeneration (more on that below). Some growing pains existed in GM's first go-round of equipping the Duramax with a DPF, as many of the units used in LMM applications (’07.5-’10 Silverado and Sierra HDs) were prone to cracking and leaking.

To keep the DPF from becoming chock full of particulate matter, a process called regeneration takes place, which effectively incinerates most of the soot buildup within the DPF. There are two forms of regeneration: active and passive. Passive regeneration occurs when the engine is producing sufficient heat to keep particulate matter low, such as in heavy towing or hauling situations. During active regeneration (commonly required on trucks that idle a lot or aren't worked hard), the ECM calls for fuel to be injected on the engine's exhaust stroke. This extra fuel is used to increase exhaust gas temperature in both the DOC and DPF to more than 1,000 degrees F in order to burn off the soot in the DPF. However, the fuel required to perform an active regeneration cycle (which occurs about once every fillup or roughly every 400 miles traveled) is the primary reason why the LMM-powered trucks took a hit in fuel economy.

The active regeneration process is triggered when a pressure difference between the inlet and outlet of the DPF is observed, but you won't notice any regeneration initiation message on the dash, unless there is a problem. To keep tabs on the truck's regeneration status, many owners turn to the Insight CTS2 from Edge Products (the original CTS monitor is pictured above). The color touchscreen monitor allows you to see when the truck is in regeneration mode, as well as watch the soot accumulation reading from the DPF (measured in grams on the bottom right). During regeneration, you’ll also notice EGT climb above 1,000 degrees, a difference in the engine's idle and that injection timing has been retarded.

In addition to meeting the EPA's new particulate matter standard, NOx emissions had to be reduced by 50 percent. This meant more exhaust gas recirculation (EGR) would be necessary on the LMM, which called for a larger EGR cooler for increased cooling capacity and operational lifespan. The square style EGR cooler used on the LMM is fairly robust compared to what you’ll find on other diesel engines, but they are known to plug up, crack and leak from time to time. The first step in trouble-shooting a leaking EGR cooler is often noticing that the engine is consuming coolant.

The name of the game in meeting particulate matter emission standards is to maintain sufficient heat in the engine. This means the engine needs to be under some sort of load at all times, and the intake airflow valve (i.e. throttle plate) shown above allows the LMM to do just that. During instances of towing, hauling or spirited driving its services aren't required. However, at idle, in stop-and-go traffic and cruising on the highway, EGT tends to drop considerably. It is here that the intake airflow valve (commanded by the ECM) is used to restrict the amount of incoming air, thereby controlling combustion temperature more precisely.

Although they retained the same basic body architecture as the units found in the LBZ and still saw 26,000 psi worth of fuel pressure, the Bosch solenoid style common-rail injectors in the LMM were equipped with revised nozzles. Specifically, a six-hole nozzle with a 159-degree spray angle on top of the piston was employed, whereas the LBZ injector had used seven-hole, 158-degree nozzle. The same reliable Bosch CP3 high-pressure fuel pump was used for pressure creation—and it was still void of a lift pump supplying it fuel from the tank.

Like the LBZ that came before it, the LMM uses a Bosch EDC16-based ECM, but it's not the same unit that controlled its predecessor. The version aboard the LMM has slightly different internals due to its need to control the new emissions system, and it also communicates with the rest of the modules on the truck through an updated CAN bus system.

To cope with the added heat (i.e. stress) the new DPF system and more active EGR system would undoubtedly produce, GM revised its cast-aluminum cylinder heads to better optimize cooling. In a direct comparison with the LBZ heads, the only real difference exists in the coolant passages. With the cylinders below them capable of seeing exhaust gas temperatures hotter than 1,300 degrees F in stock form, the ability to dissipate transitional heat more effectively was a big priority for GM engineers.

Moving the same 16.8:1 compression, cast-aluminum pistons up and down in its bores as the LBZ, the LMM's biggest weak link is also its pistons. Heat, added cylinder pressure, abuse and a lack of meat in the wrist pin region all contribute to these babies fracturing across the center line of the wrist pin, typically when power levels exceed 650rwhp.