The 5 Most Common LMM Duramax 6.6L Engine Problems

GM launched the LMM in 2007.5, replacing the LBZ due to tighter emissions regulations. To strengthen emissions active regeneration was added alongside a DPF system. As is common with emissions equipment, this led to a new common problem for the Duramax LMM engine.

Similar to its predecessor, the LMM has very few actual engine problems. With the exception of the DPF system problems, the majority of big issues with the LMM, such as cracked pistons and snapped crankshafts, only tend to occur when it is heavily modified and tuned.

Outside of these problems the only things the LMM suffers from is leaking transmission lines and occasional problems with injectors or the injection pump.

Duramax LMM Engine Problems

1. DPF system & active regeneration
2. Leaking transmission lines
3. Fueling / low fuel pressure
4. Piston / crankshaft failure
5. Allison 1000 power limitation

The LMM is an extremely capable engine. Two of these common problems are really cheats by me because there aren’t a lot of common problems on these engines and drivetrains. The DPF system is the biggest pain point and leaking transmission lines are a guarantee at some point. Outside of these two issues, these engines only become problematic when you being modding and tuning them with excessive power.

I’ll discuss overall reliability further down, but all-in-all the LMM is easily good for 300k+ miles – just don’t expect the DPF to make it that long.

If you would rather consume this content via a video, check out our Chevrolet LMM Duramax Common Problems video below:

1. DPF Failure

With the evolution of diesel emissions standards, the LMM added a diesel particulate filter or DPF. The DPF is part of the exhaust system and is responsible for capturing and removing diesel particulate that is created by the burning of diesel fuel. As you drive your diesel, the DPF captures the particulates which then are stored inside the DPF system.

The LMM has a second part of the DPF system called “active regeneration” which is what burns away all of the particulates held by the DPF so that the system doesn’t get clogged. The truck therefore has “regeneration cycles” which usually occur every 700 miles or as soon as the DPF accumulates 44 grams of soot.

Regeneration takes approx. 40 miles of highway driving. With city driving, the exhaust usually does not get hot enough to go through a full regen process. Therefore, these LMM engines suffer from excessive regeneration which causes all sorts of engine and DPF problems. Additionally, they can easily become over-clogged which can prevent the regen cycles from working. The regen process puts backpressure on the engine which can lead issues with the cooling system, fueling system, etc.

Fortunately, the majority of the time DPF problems arise, the system just either needs to be cleaned or replaced. Unfortunately, the system is expensive to replace.

DPF Failure Prevention

The DPF system is extremely complicated but all problems boil down to excessive regeneration or regeneration interruption. These problems can be prevented by ensuring you are driving your Duramax on the highway enough and being cautious of not interrupting the regeneration process while its taking place. Using a fuel additive such as AMSOIL Diesel Concentrate can help the fuel burn cleaner and therefore reduce particulate build-up.

2. Leaking Transmission Lines

Similar to the LBZ’s transmission line problems, the transmission cooling lines are also highly prone to leakage on the LMM Duramax with the Allison 1000 transmission. The cooling lines circulate coolant throughout the transmission to prevent overheating. When lines leak, the transmission can get low on coolant which results in transmission overheating and potentially serious damage to its internal components.

The OEM LMM cooling lines have a fatal design flaw which causes them to leak at the crimp. At first, it is most common for fluid to only leak when it is cold outside or the engine is cold. However, initial leaks will usually turn into consistent leaks which will put you at risk of running too low on coolant. We’ve seen these lines go bad on trucks with less than 25,000 miles and have seen folks have to consistently replace these lines every 30-40,000 miles.

Symptoms

• Overheating transmission temps
• Red fluid dripping underneath the car
• Hard shifting

How to Fix Leaking Transmission Lines

Now that these trucks no longer have factory warranty, your best option to fix leaking transmission cooling lines is to upgrade to aftermarket lines. Upgraded lines are usually slightly more difficult to install since they are made of a less flexible material compared to the OEM lines. However, the benefit is that these upgraded lines are leak-proof for the most part, making the upgrade well worth it.

Upgrade Option: Deviant 1/2″ Transmission Cooler Lines

3. Low Fuel Rail Pressure

While not extremely common, low fuel rail pressure codes and reduced engine power are a noteworthy mention. Getting this engine code is most common in summer months with hot weather when you are towing very heavy (usually 5th wheel towing 15,000+lbs.) with an older, high-mileage truck.

When you are getting a P0087 code and reduced power output, the most common problem tends to be either the fuel injectors or the fuel pump itself. Overtime, both of these parts are prone to normal wear and tear which can reduce fuel rail pressure. Injectors can get clogged or the fuel pump can simply weaken. As mentioned, this is most common when its hot and when towing heavy which requires the engine to work harder and therefore more fuel to be used.

Low Fuel Rail Pressure Symptoms

• P0087 engine code
• Reduced engine power
• High exhaust gas temps (EGT’s)
• Poor performance and shifting

Common Causes of P0087

• Bad fuel injectors
• Failing CP3 fuel pump
• Clogged or dirty fuel filter
• Leaking fuel lines

Replacement and Prevention Options

The first step is diagnosing what the culprit is. Start by inspecting and or replacing the fuel filter. As mentioned, its most commonly caused by the injectors or the pump itself. One prevention method is to install a lift pump which will reduce the stress and pressure put on the CP3 injector pump.

A second helpful option in addition to installing a lift pump is adding an ECM tune, such as an EFI Live tune. The tune will alter limp mode parameters and ensure the pump is providing the adequate amount of fueling under specific load conditions. My recommendation is first starting with a tune and a lift pump and then replacing the injectors or pump if you continue to run into this problem. Injector replacements are very expensive and a tune and lift pump are great reliability mods even if you end up having to replace injectors anyways.

4. Piston & Crankshaft Failure

While the Duramax block used in the LMM is a highly capable and reliable engine, it does suffer some limitations. If you are driving a stock LMM and have no intentions of modifying it, then you can ignore this issue. I’ve never heard of the pistons cracking or the crankshaft snapping on stock engines.

However, for those looking to push additional power, it’s worth noting that the LMM is prone to piston and crankshaft failure at high horsepower levels. Around the 600whp mark is when this becomes a very likely problem. You’ll notice this is also an extremely common problem on tuned LBZ’s. Given the similarity between the two engines, it’s no surprise the LMM is prone to this same issue. While the engines are nearly identical with the exception of the emissions systems, the LMM’s pistons tend to fail more easily and at lower power levels compared to the LBZ.

The LMM has a 6-hole design, where two holes spray fuel directly above the wrist-pin on the piston which creates excess heat in this area. Unfortunately, the wrist-pin is the weakest part of the pistons so the excess heat can easily cause them to crack.

5. Allison 1000 Power Limitations

You’re probably only cracking a piston or snapping a crankshaft with either a very aggressive tune, or an upgraded turbo among other modifications. Unfortunately, the same cannot be said for the Allison transmission. While these transmissions are ultra-reliable workhorses, they do not handle additional power very well.

Running a modest tune with an intake and exhaust probably won’t give you much trouble, but once you add ~100+hp to your LMM, the Allison becomes a weak spot. However, any additional power is going to take a toll on your transmission, whether it’s just a modest tune or a heavily modified engine.

If you’re going to add an engine tune we certainly recommend also adding a TCM (transmission control module) tune to help strengthen the tranny. With a TCM tune, the Allison can handle up to 450whp for extended periods of time, with 500whp being the breaking point. 

Duramax LMM Reliability

Consistent to its very similar LBZ counterpart, the LMM is a highly reliable engine. The DPF is a hot-spot for failure so beware that you will probably end up with a few thousand dollars of DPF/emissions repairs.

Without the DPF and with upgraded transmission lines, you’re unlikely to run into very many seriously costly repair bills on the LMM. As with all things that age, you should expect to run into some general maintenance items once you surpass the 150k mile mark with things like injectors, the water pump, the fuel pump and potentially the turbo.

Overall, there is no reason that a stock LMM can’t make it to the 300k mile mark reliably.