Following the short lived LLY engine, the Duramax LML was introduced in 2011. Featured in the same 2500 and 3500 HD work trucks, the LML sported the same 6.6L turbo-diesel design that Chevy diesel trucks have been using since 2001. However, the LML was actually the biggest overhaul Duramax had done to the 6.6, claiming to use nearly 60% of freshly designed parts not used in previous version.

Frequent across all diesel engine manufacturers, the engine update was primarily driven by increasing diesel emission standards. While the prior LMM had added DPF, or diesel particulate filters, the LML had two new, major emissions upgrades with the addition of selective catalytic reduction (SCR) and diesel exhaust fluid (DEF). Due to these advanced emissions systems, the LML had the longest production history of the 6.6’s to-date, lasting until 2016 after which it was replaced with the L5P.

At 400hp and 765tq, the LML is also the highest power producing Duramax made to-date, up until the L5P.

Duramax LML vs. LMM Improvements

Because the LML had some major changes, I want to briefly cover its differences compared to the LMM.

Block, Rods, & Pistons

First, the engine received a strengthened block and improved main bearing to improve on previous weaknesses. Additionally, the rods and pistons were both strengthened while actually being lighter, creating a lighter rotating assembly. While the rods and pistons are both made similarly and out of the same material as the LBZ/LMM rods and pistons, the piston wrist pins had modified ends for more support and the rods featured a cracked cap design. All of these changes significantly improved the durability and strength of the rods and pistons over the previous versions.

With the improved rods and pistons and lighter rotating assembly, cracking is no longer a common issue as it was on LMM and LBZ models and the internals can now handle up to approx 700rwhp without any issues.

Injectors & Injection Pump

The LML included an advanced injector design, capable of 30,000psi of pressure compared to 26,000 on the LMM. The increased pressure capacity allows the engine to produce the significant power.

To support the higher pressure injectors, the LML was fitted with the Bosch CP4.2, upgraded from the previous Bosch CP3. Despite the upgraded injection pump, it actually flowed 20% less fuel than the CP3, creating problems with performance modification and tuning. Additionally, the injection pump does not take well to contaminates and poor lubrication, causing a myriad of potential problems which we’ll cover later on.

DPF & a 9th Injector

The DPF regeneration process was overhauled by adding a 9th injector in front of the diesel oxidation cat which is located within the turbo downpipe. The 9th injector receives fuel from the CP4.2 and injects it directly into the exhaust system. This removed the oil dilution issues with the LMM engine and increased the regeneration intervals.

DEF & SCR

Selective catalytic reduction was added to the engine, which injects a special fluid, known as diesel exhaust fluid into the exhaust system. The exhaust fluid creates a chemical reaction which convers nitrogen oxide (the bad stuff) into nitrogen, water, and carbon dioxide. While these systems further complicate the broader emissions process, they actually improved fuel economy by about 10%.

The drawback is that you now have to also fill your Duramax engine up with diesel exhaust fluid on a regular basis, although it is relatively inexpensive and offset by the fuel economy gains.

The 4 Most Common Duramax LML Engine Problems

• CP4.2 Injection Pump Failure
• DEF Heater Problems
• NOx Sensor Failure
• DEF Pump Failure

The buzzkill of the LML is the CP4.2 injection pump. Outside of this problem, there aren’t many major or costly common problems associated with the engine. Fortunately, there are some relatively simple ways to prevent these issues and make them less common.

Duramax LML CP4.2 Injection Pump Failure

Duramax “upgraded” the injection pump for the LML models, going from the Bosch CP3 to the CP4.2. To handle the additional power capabilities of the engine the fuel rail pressure was increased to 30,000psi. However, the pump actually flowed 20% less fuel than its CP3 predecessor. This makes the CP4.2 highly incapable of handling additional horsepower without the addition of a lift pump.

But, added power isn’t the achilles heel of the CP4.2, bad fuel is. The fuel pump has two piston-like components inside of it and it only uses diesel fuel for lubrication. Diesel fuel is highly susceptible to picking up water, dirt, and other contaminates. Because neither water nor dirt are good lubricants, these getting into the fuel pump can wear down the internal components and cause disastrous damages. Once one small shaving starts circulating, a chain reaction begins and the CP4.2 ends up destroying itself from the inside, while taking the whole fuel system out with it.

Small aluminum bits can shave off of the internals of the pump and then get circulated through the whole fuel delivery system. The fuel rail, the injectors, fuel lines, etc. all will have metal shavings circulating throughout. Once this happens, the fuel pump will fully die, leaving your engine starving of fuel.

The injection pumps on the LML usually fail with little to no warning, and total failure only takes a few minutes once the first symptom occurs.

Injection Pump Failure Symptoms

• Poor performance and idling
• Cylinder misfires
• P0087, P0088, P0191 or P128E engine codes

Replacement Options

When the LML’s CP4.2 injection pump fails, Chevrolet recommends replacing:

• Injection pump
• Fuel rails
• Fuel injectors
• High-pressure fuel delivery lines
• Low-pressure return lines
• Clean out fuel tank, sending unit, and all main lines

As you can imagine, this is a very expensive job. A failed CP4.2 injection pump in an LML can be a $10,000+ repair job for all parts and labor. Because the primary cause of failure is bad diesel fuel, this can happen on an LML with 10,000 or 200,000 miles. Additionally because of this, adding a lift pump won’t solve the problem either (although it will solve the added power problem).

Most owners consider the injection pump a ticking time bomb and the best “replacement” option is just to swap it out for a different injection pump before it breaks.

Diesel Fuel Quality

Always use the best diesel fuel. Even if you don’t have the CP4.2 injection pump on your truck anymore, diesel fuel is still extremely important. Bad fuel will jack up your injectors and cause your engine to run poorly.

Use a high quality fuel additive to help treat diesel fuel – we recommend Stanadyne.

Try to fill up on fuel from gas stations that get a lot of traffic and sell a lot of fuel. Diesel fuel sitting in tanks for months because it never gets bought is an easy way for it to accumulate excess dirt and contaminates.

Also, replace your fuel filters frequently.

2. DEF Heater Problems

One of the emissions additions to the LML was the inclusion of a selective catalytic reduction (or SCR) system as mentioned at the start of this article. SCR requires diesel exhaust fluid “DEF” to operate properly. Therefore, the LML has a separate DEF fluid tank in additional to the normal diesel fuel tank.

DEF fluid consists of about 67.5% water, with the remaining being urea. Due to the high water content, DEF fluid is prone to freezing. Frozen DEF doesn’t quite work very well so the system is accompanied by three heaters. One in the fluid tank, one in the DEF pump, and one in the supply line which delivers the fluid to the injector.

When DEF fluid gets to a certain temperature, the temperature sensor alerts the glow plug control module (GPCM) to kick the heaters on.

The DEF heaters fail frequently, sending the engine into limp model with reduced power.

Engine DTC Codes for Failed DEF Heater

• P21DD
• P20B9
• Limp mode (will appear on heads up display)

Replacement Options

There are three heaters located through the DEF system and all are prone to failure. The only replacement option is to replace the heater, unless you want to bypass the code and limp mode. For some individuals who live in very warm climates where the temperature never drops below freezing, the DEF heater is pretty irrelevant.

However, whether you need the heater or not, a failed heater will throw codes and put the car in limp mode. If you don’t want to spend the $300-$500 replacing the heater there are a number of ways to bypass the code. Alternatively, completely removing the SCR & DEF systems is somewhat common, although illegal per emissions laws.

3. NOx Sensor Failure

The goal of DEF and the SCR system is to reduce the amount of nitrogen oxide that enters the atmosphere via exhaust gases. The system has two NOx sensors: one before the DEF injection location and one after. The goal of the NOx sensor is to ensure that the fluid is adequately reducing nitrogen oxide levels.

Specifically in 2011 LML’s, the NOx sensors had a design fault which caused them to either fail or lose calibration with the ECM.

Poor DEF Quality Fault Warning

The engine computer takes the reading from the first NOx sensor and compares it to the reading on the second sensor. When the amounts of NOx have not been adequately reduced from sensor 1 to sensor 2, the system assumes there is an issue with the DEF. When the NOx sensors lose calibration, the NOx readings are not properly relayed to the engine’s computer.

The result of a faulty sensor is receiving a “Poor DEF Quality” message. The message will give you a certain amount of miles to resolve the code otherwise it will send your engine into limp mode. Other common NOx sensor symptoms are these engine codes:

• P20EE
• P207F

The sensors do not measure DEF quality! Therefore if you receive this code, simply replacing your DEF fluid will not cut it.

You will need to take your LML to a Chevy dealer or to someone with the proper software to correct the issue. The sensors will need to be recalibrated to the engine’s computer which requires specific GM/Chevy diagnostic equipment.

4. DEF Pump Failure

If you’ve noticed a trend with the LML, it’s that the DEF systems are problematic. This was new technology at the time and was bound to have its fair share of issues. On the LML, the DEF system is equipped with a typical pump. The pump sends the DEF fluid from the reservoir tank to the SCR system located in the exhaust.

As with any fluid pump, it’s prone to failure via normal wear and tear, dirt and contaminates in the fluid, etc. Failure on the LML’s hasn’t exactly been pinpointed but the pumps have been known to fail as early as 30,000 miles.

When the pump fails, no fluid gets sent to the SCR system. Therefore no NOx reduction takes place, causing the NOx sensors to give readings in the same range. This causes the “bad DEF quality” light to come on which will again give you the mileage countdown until limp mode.

LML DEF Pump Failure Symptoms

• DEF light illuminates
• Bad DEF quality message

The replacement options here are either to delete the DEF system or replace the pump. The pump is located within the reservoir tank.

Duramax LML Reliability

The biggest concern on any LML is the CP4 internally combusting. The associated cost of this is about $10k and it’s not covered under warranty because it is caused by poor fuel. Various lawsuits have been filed here and there might be a way to receive some type of compensation, but for the most part this is your bill to bite.

However, the CP4.2 failing is usually debated to be “not as common” as people make it out to seem. The commonality of this has certainly been somewhat inflated due to the cost of the repair. But, any problem that can occur out of nowhere that costs $10k is going to get a lot of attention. The best peace-of-mind option here is to do a CP3 conversion.

Outside of the injection pump, the LML is a reliable engine with the exception of the emissions systems. DEF and SCR cause their fair share of problems, but are also covered under warranty for the trucks that still have it. The problems are common enough that deleting the DEF system is frequent among LML owners. But do note this is technically illegal.

Beyond emissions systems and the injection pump, the LML is the strongest Duramax engine built to date. The block, rods, and pistons on prior Duramax engines were common failure points on tuned and modified engines due to weak components. All of these items received upgrades in the LML. With proper preventative maintenance, the LML engine itself is easily capable of 300,000 to 400,000 miles.