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Everything You NEED to Know About the LMM Duramax

Everything You NEED to Know About the LMM Duramax

The LMM came about in response to a 2007 EPA mandate that extended emissions requirements to the diesel engines used in light trucks with the objective of reducing nitrogen oxides and particulate matter. In order to comply with these new regulations, GM responded by installing new emissions control devices such as a diesel particulate filter (DPF), an intake airflow valve, and a larger exhaust gas recirculation valve (EGR). While they were not received well by the aftermarket community as it made tuning these vehicles more difficult, the new components significantly reduced the level of contaminants that were released into the air.

With that said, the LMM was slightly more powerful than the LBZ, putting out an extra five horsepower and 10 ft/lbs. of torque. Like its predecessor, the LMM Duramax was a sturdy motor, responded well to modifications, and could handle up to 600 horsepower before things started to break.

Origin Story

The LMM could be summed up as an LBZ with some added emissions control equipment as it was practically the same engine aside from the coolant passages in the head being revised to handle the extra heat.

Other minor changes came in the form of a six-hole injector (previous models had seven) that sprayed fuel directly on the top of the piston for a more complete burn. Other than that, the fuel pressure and the reliable Bosch CP3 pump remained unchanged. The trustworthy Allison six-speed transmission received some minor improvements and was noted as being sturdier and able to handle more power (and abuse) over the one installed in the LBZ.

Where the divergence occurred was in regards to the new emissions control equipment. While the LMM received a minor bump in horsepower and torque, it took a step back in terms of fuel economy thanks to the DPF. Not to mention, it introduced some new problems in the form of reliability that took a few years to work out and that brings us to the next question.

What is a Diesel Particulate Filter?

While the DPF had been used in commercial vehicles prior to 2007, this was the first year it saw widespread use in light trucks like the Silverado 2500/3500. We’ll spare you the long technical discussion of how it works, but basically the DPF traps (and burns off) excess particulate matter. This is done via two methods with the first being known as passive regeneration and this occurs when the engine is running at high temperatures (like when towing or hauling).

The second process is known as active regeneration and this is achieved by adding extra fuel to the exhaust stroke, thereby raising the exhaust temperature and burning off the soot that has built up in the filter. This typically happens every 700 miles or when the filter has accumulated 40 grams of soot. It should be noted that active regeneration was largely responsible for the decrease in fuel economy.

All said and done, the LMM still managed to put down 365 horsepower and 660 ft/lbs. of torque, more than anything Dodge and Ford had to offer at the time. While the new emissions control equipment limited tuning options (albeit they could be removed) the LMM had plenty of potential.

LMM Engine Specifications & Tech Dive

Production YearsMid-2007 to 2010HeadsCast Aluminum
Design90 Degree V8ValvetrainOHV, 4 Valves per Cylinder
Bore4.055 in. Injection SystemBosch High Pressure Common Rail
Stroke3.897 in. InjectorsBosch 6 hole
Displacement403 ci (6.6L)Injection PumpBosch CP3
Compression Ratio16.8:1TurbochargerGarrett GT3788VA
BlockCast IronIntercoolerAir to Air
RodsForged SteelHorsepower365
PistonsCast AluminumTorque660

Block

The LMM retained its predecessor’s upgrades such as the added webbing in the areas of the main bearings and taller main bearing caps with a 4mm deeper bore for the cap bolts.

Overall, the block has never been regarded as a weak spot on the Duramax engines. It is more than capable of handling some serious power and other components like the pistons, connecting rods, and crankshaft will give out long before you need to worry about the block. There would be further improvements in 2011 with the introduction of the LML.

Rotating Assembly & Top End

The larger forged connecting rods and piston design of the LBZ were carried over as well. Unfortunately, that included the issue of cracked pistons at higher power levels, and this persisted throughout the entire production run of the LMM.

While the heads were the same for all practical intents and purposes, the coolant passages were redesigned to optimize cooling and handle the extra heat caused by the new emissions control equipment. All said and done, this was the only thing that was changed internally and the LMM had the same strengths and weaknesses as the LBZ.

Turbo & Fuel System

Like the engine, there were only subtle changes to the fuel system. The tried-and-true Bosch CP3 common rail pump and 26,000 psi injectors were carried over albeit with a minor change to the latter. They now had six-holes as opposed to the previous seven-hole design on the LBZ. In addition, the spray pattern was optimized, and it was injected directly over the center of the piston.

On the turbo side, the LMM used the same Garrett GT3877VA, and it was more than up to the task of handling higher boost levels offered by performance tunes.

LMM Reliability

Engine-wise, there are no major issues with the LMM as it was practically the same as the LBZ. With that said, the overall reliability of the truck it was installed in could be regarded as a step back due to the problems associated with the DPF and it took a few years to iron them out.

It has been said the increased operating temperatures of the LMM can reduce the longevity of the engine and related components. While there is some truth to this, much of it can be attributed to maintenance (or lack thereof).

Problem 1

Diesel Particulate Filter

Unsurprisingly, the DPF was not exactly a popular feature, and it was redesigned in 2011. Aside from causing a drop in fuel economy during active regeneration, (when extra fuel is injected into the exhaust) the accumulation of incomplete cycles frequently causes the filter to become clogged or fail altogether.

This had to do with the fact that active regen normally required the vehicle to be operated at highway speeds and it took about 30 minutes to complete. As you might guess, this was a costly component to replace with many people opting to remove it altogether.

Problem 2

Cracked Pistons

This should come as no surprise to anyone familiar with the LBZ as cracked pistons were a problem in that motor as well. It had to do with the design of the wrist pin bushings which were known for giving out at higher power levels. However, it is reportedly more common in the LMM due to the injector design that fuel sprayed directly over the piston’s center, thus subjecting the wrist pin to more heat.

Suffice to say, this rarely occurred in stock engines or those with mild tunes and it did not really become an issue until the 600hp mark.

Problem 3

Leaking Transmission Lines

While the Allison 1000 is a stout transmission and it received some upgrades in 2007, the coolant lines frequently leaked due to a poor crimp design and it was a known problem on the LBZ well. 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.

Trust The LMM Duramax Experts

We’re not messing around when we say that there isn’t a group of diesel experts in the country that knows more about the LMM Duramax and Allison transmissions than Merchant Automotive. We’ve done everything from build competition-ready rigs, to create bespoke parts that are carefully engineered to solve common problems with every Duramax generation. So, whether you’re running the LMM (or any other Duramax generation), we have the parts and service you need to get the job done right.

So what are you waiting for? Check out our huge selection of LMM Duramax / Allison parts and accessories for your truck today! 

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