The 5 Most Common LMM Duramax 6.6L Engine Problems
Jake is a founder of 8020 Media and has been creating automotive content online since 2017. He has been the lead writer for Chevy Trucks and has transformed it from the old and outdated site it was into what it is today. Jake creates a ton of GM related content for the 8020 Media YouTube channel and specializes in Duramax and Vortec information but has a wealth of knowledge across all GM cars and engines. Jake believes the L5P is the best diesel on the market today.
Following the LBZ being phased out in mid-2007, the LMM Duramax took its place as Chevy & GMC’s diesel truck engine. Following the same 6.6L powerplant, the LMM was introduced due to continuously increasing diesel emissions regulations and lasted until 2010 when it was phased out for the LML.
Compared to the LBZ, the LMM added active regeneration cycles to the diesel particulate filter (DPF) system. Outside of this complex emissions system, the LMM remained very similar to its predecessor. While the power output was increased to 365hp and 660lb-ft of torque, its fuel economy suffered materially due to the new DPF system.
Paired with the trustworthy Allison 1000 transmission, the LMM actually outperformed both Ford’s 6.4L Powerstroke, and Dodge’s 6.7L Cummins of the time. You can read all about the LMM’s differences compared to the LBZ here. Similar to its predecessor, the LMM has very few actual engine problems. Outside of tuned LMM’s, most problems occur with ancillary engine systems rather than major engine components themselves.
5 Most Common LMM Duramax Engine Problems
- DPF system & active regeneration
- Leaking transmission lines
- Fueling / low fuel pressure
- Piston / crankshaft failure
- 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.
If you would rather consume this content via a video, check out our Chevrolet LMM Duramax Common Problems video below:
1. LMM 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.
LMM Leaking Transmission Line Symptoms
- Overheating transmission temps
- Red fluid dripping underneath the car
- Hard shifting
How to Fix Leaking LMM Duramax 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. LMM 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 – LMM Duramax
- 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 LBZ has a 7-hole injector system, while the LMM only has a 6-hole injector. With the 6-hole design, there are two holes that 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.
While the Allison 1000 was also a problem on the LBZ (do you notice a common theme here?), the one equipped in the LMM Duramax’s can actually withstand a bit more torment. 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. I would point out that you should not expect my “for an extended period of time” comment to mean 150k miles.
Power levels above that and you’re going to want to build out the transmission and upgrade some of its internal components.
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, however you can *illegaly* remove this potential problem by removing the DPF system.
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. If you follow a proper maintenance schedule, changing fluids as recommended, this engine will be a fantastic work horse or toe rig. Just note that the more frequently you tow very heavy items, the more quickly repairs and maintenance will be needed.
So glad I found this , I have an 07 lmm and replaced the tranny line that was leaking , now the other one is leaking. I put a runner on and deleted the egr , going to put a better torque converter in and flywheel , also 5″ exhaust. And change to 410 gears cause I’m putting 37″ tires on , when I can afford the lift . With all this done hopefully it will be bulletproof , I’m sure I’ll find more to do . So glad I found the information about my LMM ,
I’m running 4.88:1 gears with 37s on my ‘08 Dmax. If you use a tire to gear ratio chart you’ll see that between 4.56 and 4.88 is where the equivalent to stock is. I think 4.11 will still seem too high. Just an FYI
I think it’s worth noting that the water pump is also an issue as it’s pressed and not “welded” will cause the impeller to slip over time.
We’re looking for an 6.6L Duramax LMM, 90 degree V8 diesel, for our Hummer H1 fully converted.
if you can provide us the needed engine, please send us a Quotation pdf.
Do you still need a engine?
im currently having a program issue with m,y 08 duramax , just replaced transmission , truck keeps going into limp mode ,question is , would it be safe to reprogram to gm spec;s to get truck running again to fix problems then reinstall m y efi would their be conflict between the two programs
I have an 08 LMM 3500hd dually mini-maxx tuner. I tow a 16k 5er almost exclusively all over the country. Search intercooler to avoid egt too high, trans pan adding more fluid and cooling, lift pump and two filters, rear end cover Banks, all synthetic fluids
At 75k a piston cracked as two Injectors were bad. I’m my own mechanic and spent 14k rebuilding with long block(race motor). Never exceeded tune 2, towing. Estimate 420hp. Just saying
I have 2008 6.6 LMM now i start it run for 5 second and stop I have to breeze it to it run again what could do I have to put lift pump
I have a 07.5 lmm I added a bully dog. Upgraded my trans cooling lines with the heavy hydraulic like hose. Having problems with map and mas sensor’s removed the resonator. 144000 miles otherwise bone stock. Lots of torch and hp Exhaust break gooseneck class 4 receiver and airbags.
Can I put a chip in my 6.6 LMM duramax 2007 and still pass smoke, or better yet what can I do and still pass smoke In California
Smog not smoke
I have a 2009 duramax GMC crew cab 4×4 sierra ltz. I did a complete emission delete back 2014. Awesome truck. Never had any issues except normal wear items like U joints flued change belt, wheel hubs shocks. Stay up on the fluid changes and drive it halfway nicely and you will get 22mpg on the freeway and 14 around town. Never had any electrical issue. never needed to change the alternator or anything under the hood except 3 glow plug shafts, 15 bucks a pierce and a 5 min change out per. Pulled med sized loads up to 8k- 12k trailers. hauled plenty in the bed. I did upgrade the headlights and bed lights to LED made a huge difference, 70 bucks for good ones on amazon, Leather interior shows a bit of wear in the seat areas and arm rest, steering wheel. It was never a dedicated work truck though mind it def could be with some nice seat covers. The interior is gadget free gets the work done and has a nice mid 2000s Bose cd player, aux input and sound dam good. I also did a leveling kit back 09 and hasn’t been an issue. I just turned 214K miles this week. Never leaked a drop of fluids.
I forgot to add I run a H&S mighty max tune to bypass emissions and only drive it in tow mode the street and compotation mode are way to much stock engine and trans.
I have 2007 gmc sierra 2500hd diesel duramax 19000 miles i bought from single owner I am second owner its works fine put new transmission cooler and new oil valvoline synthetic in the city 15.5 to 16.7 one down on that is 26 gallon tank
My son passed away in May and he owned a 2008 Duramax. In March of 22 it was stolen.. they stole all the keys, took of in the Duramax and ran it out of gas, stole the radio.. or navigation (whatever was there). Will I be able to put gas in and it start? What should I do if it won’t start? I’m in Knoxville TN. I have to sell everything to put cash in Estate Account..I’m trying to get through this for his daughter, my granddaughter.
Terri – sorry for your loss and the added inconvenience of having the truck stolen. I’d recommend putting some fuel in it (just remember to use diesel and not normal gas) and go from there. Unless the thieves jacked with other parts of the truck then you shouldn’t have any issues with it starting. If it doesn’t start let me know what happens when you try and I can point you towards a few things.
If you don’t have a key then you have a few options to start it…you can either jump it at the starter or sometimes just jamming a screwdriver into the ignition will allow you to turn it. A mobile mechanic can probably fix the ignition and get you a new key for a relatively affordable price.
I’ve got a 2008 GMC LMM Duramax with only 100,700 miles owned since 2011, It’s an extended cab full load with after-market stereo, rear sway bar, Bilstein B6 shocks, extra large capacity trans pan, and secondary engine oil filtration system (KN). Recently chipped (moderate) removing ERG system increasing power and mileage. Change all fluids regularly and only synthetic. Will do trans lines soon. Absolutely love the engine! Best reliability of all out there’. What no one mentioned it is only American diesel truck with independent front suspension great for handling!
I have 2008 crew cab 4×4 352000 miles fuel pump and alternator other than the norms
I have a 2008 and a 2016 Duramax and both have had a broken crank. I run stock. The article said they haven’t heard of any stock broken cranks, but I have had two for two. A mechanic account of mine said he looked into it and the front two cylinders fire one after the other and the crank won’t take it.