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Everything You NEED to Know About the LML Duramax

Everything You NEED to Know About the LML Duramax

Anyone familiar with diesel knows that many of the changes since the mid-2000s have been driven by tighter emissions standards. Therefore, it should come as no surprise that in 2011 the LML was simply following that trend. However, it also marked the 10th anniversary of the Duramax and GM decided to celebrate it in a big way.

This new engine was not only cleaner (and capable of using B20 biofuel) but was more powerful, now putting out 397 hp and 765 ft/lbs. of torque (up from 365 and 660). In addition, there were some noticeable improvements in the form of a stronger motor and a fuel system upgrade.

What does all of this mean? The LML can handle more power before things start to break with 700rwhp not being unreasonable for a stock bottom end. Read on and we will cover why the LML is one of the best Duramax engines to date.

Origin Story

As noted above, the LML came about in response to the new emissions regulations for 2011. Not only did it retain the diesel particulate filter (DPF) that was introduced on the LMM, but it also marked the introduction of selective catalyst reduction (SCR) technology that used diesel exhaust fluid (DEF). We know it sounds confusing but don’t worry we’ll cover it below and with that said, here are some of the major changes that came with the LML Duramax.

The Engine

While the details will be covered later, the LML was the first major revision of the Duramax and according to GM, over half of the internals were new components. Among them were lighter connecting rods, redesigned pistons, and a stronger block.

New Fuel Pump and Injectors

Other changes came in the form of a higher-pressure Bosch CP4.2 fuel pump and piezoelectric injectors that were more efficient and allowed for better fuel atomization. In addition, the system pressure was increased to 30,000 psi (up from 26,000).

An Improved Regeneration System

Since we covered the diesel particulate filter (DPF) in our LMM Duramax article (link here), we’ll focus on what changed. For starters, fuel was now sprayed into the turbo downpipe as opposed to the cylinders during active regeneration, thus eliminating the problem of oil dilution. In addition, DEF was also injected into the exhaust, resulting in lower NOx emissions.

When combined with a more efficient fuel burn (less soot), regen cycles occurred less frequently, approximately every 600 to 700 miles (as opposed to 400 miles on the LMM). As a result, fuel economy is said to have improved by about 10%.

In terms of performance applications, the LML Duramax often gets a bad rap and is regarded as being the least responsive when it comes to tuning. While there is some truth to that, the smaller turbo and lower volume CP4.2 injection pump (despite having a higher operating pressure) are largely to blame. Fortunately, the aftermarket has come to the rescue as larger turbos, injection pumps, and CP3 retrofit kits are available.

Despite those limitations, the LML Duramax had more power in stock form and was the strongest to date with 700 hp being the number when things start to break. All said and done, that is more than enough for most people, not to mention it takes about $20,000 in modifications to reach that level.

LML Engine Specifications & Tech Dive


Production Years2011-2016HeadsCast aluminum
Design90-degree V8ValvetrainOHV, four valves per cylinder
Bore4.06 inchesInjection SystemBosch direct injection high-pressure common rail
Stroke3.90 inchesInjectorsBosch piezoelectric
Displacement6.6L (403 ci)Injection PumpBosch CP 4.2
Compression Ratio16.0:1TurbochargerGarrett GT3788VA VVT
BlockCast ironIntercoolerAir-to-air
RodsForged steel, fractured capHorsepower397
PistonsCast-aluminumTorque765

Block

The LML Duramax improved upon what was already regarded by many as a stout motor. However, GM decided to go from better (the LBZ/LMM) to best, the LML. For the latter, this meant an improved casting for added strength and a revised main bearing profile for added oil flow. Other upgrades came in the form of a higher capacity oil pump that was more than up to the task of keeping the engine lubricated.

Rotating Assembly & Top End

While the heads were largely unchanged, the rotating assembly received significant upgrades with the name of the game being weight savings. However, this does not mean weaker and in fact, quite the opposite. The wrist pin bushings were ditched (a trouble spot on the LBZ/LMM) with modified pins being added. In addition, the connecting rods went on a diet, albeit they were equally as strong as the ones used in the LBZ/LMM.

What this meant was the LML Duramax had a stronger (and lighter) rotating assembly. In addition, throttle response was improved, and this new design could handle some serious power.

Turbo & Fuel System

The LML also marked the introduction of a new Bosch CP4.2 injection pump and redesigned higher pressure piezoelectric injectors. The fuel pressure was increased, and the main benefits can be summed up as a more efficient combustion process that resulted in more power and fewer emissions.

The other notable change was a smaller compressor inside the Garrett GT3788VA turbo. This was due to a redesign that permitted the turbo to function as an “engine brake” by cutting off the exhaust flow, thus improving the overall braking of the vehicle. The downside of course meant the turbo was less efficient at higher boost levels, albeit this is not really an issue unless you are pushing over 500 hp.

LML Reliability

Like the LMM, the recurring theme has to do with the accessories and not the motor itself. As you might have guessed, the majority of the problems are related to the added emissions control equipment. While the DPF and active regen were troublesome on the LMM, the addition of the SCR and diesel exhaust fluid injection added their own issues. Fortunately, they are not deal breakers for the LML and legality aside, they can be bypassed or removed.

Aside from the CP4.2 injection pump, LML Duramax was a reliable motor that could last 300,000 to 400,000 miles when properly maintained.

Problem 1

CP4.2 Injection Pump Failure

The CP4.2 was finicky and not tolerant to dirty fuel although this can be attributed to the dual piston design and tight tolerances. Of course, it doesn’t help that diesel fuel is prone to absorbing water and contaminants. Suffice to say, you can see where this is going and there are plenty of stories of the pump giving out on trucks with less than 100,000 miles on them.

Unfortunately, it often does so rather suddenly and without warning. Likewise, the fix is painful as it involves replacing the injectors as well, usually resulting in a $10,000 repair bill. Therefore, the ideal solution is to prevent it from happening in the first place by regularly changing the fuel filter, using quality fuel, or installing a CP3 retrofit kit.

Problem 2

Diesel Exhaust Fluid Heater

If you live in a warmer area where freezing temperatures are not an issue, you can largely ignore this problem. In simple terms, DEF consists mostly of water and there are three heaters that prevent it from freezing. Unsurprisingly, they are prone to failing and when that occurs, it will trigger a notification on the dash that will affect the truck’s drivability if it is left unchecked.

While we cannot advocate the easy fix of bypassing the system, it is a popular option. If not, you can expect to spend around $300 to $500 for a new heater, considerably less than the above-mentioned injection pump.

Problem 3

DEF Pump

Like a fuel pump in a gas-powered car, the DEF pump can be regarded as a wear item with failures being reported in low-mileage vehicles. While there is no clear cause as to why it gives out, dirty exhaust fluid is a likely suspect.

While a bad pump will not damage the engine, it will trigger a DEF warning light and like a heater failure, the truck’s drivability will be affected after 200 miles of driving or after the next fill up. Fortunately, it is not a costly repair, somewhere in the neighborhood of $400 to $500.

Trust The LML 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!

To check out our full selection of parts or to schedule an appointment contact us at:

Duramax & Allison Specialists | Merchant Automotive (merchant-automotive.com)

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