In the US, the majority of full-size truck sales go to three brands: GM, Ford, and RAM, and really it’s not even close. More specifically, when you look at heavy-duty trucks, the trucks that aren’t meant for grocery-getting or your average commute, the trucks that are specifically designed for towing and hauling big loads, those trucks are completely dominated by the aforementioned three brands.
Each truck has its pros and cons, but we’re really not here to look at these trucks as a whole; we’re here for what’s under the hood. GM has been offering diesel engines in their trucks since the early 1980s, but in 2001 they made a big shift that had a profound impact on the diesel truck landscape: they introduced the Duramax line.
So, today we’re going to go on a journey and look at every Duramax engine that GM has built, from the LB7 all the way up to L5P. We’re going to look at what makes each engine special interesting, the common problems, and why GM had to continue iterating the Duramax line with new engines every few years. But, before we can look at the Duramax engine, we need to take a look at what GM offered from 1982 to 2000, with the 6.2L and 6.5L Detroit Diesel engines.
The 6.2L and 6.5L Detroit Diesel Engines
As some of you may or may not know, GM developed a diesel engine they debuted in 1978, and that engine was the Oldsmobile diesel. Oldsmobile started with their 350 cubic inch gas engine, took the architecture, and created their diesel. Unfortunately, that diesel ended up having an insane amount of issues, and it was basically a turd.
So, when GM wanted to drop a diesel in their trucks, they took the Oldsmobile failure to heart and went directly to an experienced diesel engine builder, who also happened to be under their umbrella, Detroit Diesel. As to why they didn’t just go to Detroit Diesel for their Oldsmobile diesel engine, I’m not sure. But, regardless of that, Detroit Diesel and GM put together the 6.2L to be used in GM’s pickup trucks.
This was an iron-block, iron-head, 379 cubic-inch engine with a pretty simple 2-valve pushrod valvetrain setup. With the set of applications already very clear, it was designed to bolt up to the engine mounts and transmissions used in GM’s trucks. To call 6.2L Detroit Diesel underpowered would be an understatement. When they dropped this engine, it produced a whopping 130 horsepower and 240lb-ft of torque.
While this power figure is pretty gutless, especially by today’s standards when new trucks are producing 1,000lb-ft of torque easily, it made sense back then. Really, if you wanted big power, you would’ve opted for the 454ci big-block engine.
Today, the 6.2L Detroit Diesel is absolutely loved by truck enthusiasts for its simplicity and mechanical design. It has no electronics whatsoever, and with such a simple design, it’s pretty bulletproof. To this day, Banks still offers their turbo system for the 6.2L, and it still sells. The reason is simple: people love that old engine.
It wasn’t turbocharged or advanced in really any way. Strangely enough, it used the same DB2 mechanical injection pump as the Oldsmobile V8 diesel. They offered the 6.2L from 1982 to 1992, when they dropped a big upgrade on the diesel truck world: the 6.5L Detroit Diesel. The 6.2L and the 6.5L shared the stage through the 1993 model year until the 6.2L was fully phased out in favor of the 6.5L.
By increasing the cylinder bore from 3.98 inches up to 4.06-inches, the 6.5L was born. Interestingly enough, it was available in both turbocharged and naturally aspirated forms. The L56 and L65 were the turbocharged versions, and the L49 and L57 engines were the naturally aspirated engines. It’s also worth noting RPO codes LQN and LQN were also offered.
While the 6.5L was a big step up from the 6.2L, it wasn’t exactly up to par with what Ford and Dodge were offering. Ford had the 7.3L IDI Turbo and later the 7.3L Powerstroke, and Dodge was offering the 5.9L Cummins. Luckily though, GM was aware that the 6.2L was underpowered and the 6.5L wasn’t particularly reliable, so they worked really hard on their next diesel platform, the Duramax.
The LB7 Duramax
This brings us all the way up to 2001, when GM dropped the LB7, their first Duramax engine. This engine was a totally new slate and a far cry from the 6.2L and 6.5L Detroit Diesel engines before it. In fact, Detroit Diesel wasn’t even part of this engine. Instead, GM went to Isuzu.
What made the LB7 special upon release was the common-rail direct-inject system. This isn’t something Dodge and Cummins implemented until 2003, and it wasn’t until 2008 that we saw this feature on the Powerstroke engine.
Like every Duramax that followed, it featured 6.6L of displacement, a cast-iron block, aluminum heads, four valves per cylinder, and a single turbocharger mounted high in the center of the engine. To help fuel the LB7, GM went to Bosch and implemented their CP3 high-pressure injection pump. This injection pump maxed out around 23,000 PSI, and it was mounted in the front of the lifter valley.
In 2001, the LB7 Duramax output an impressive 235hp and 500 lb-ft of torque, thanks in part to a 17.5:1 compression ratio, but GM didn’t stop there. By 2004, the power figures grew to 300hp and 520 lb-ft. Again, to call this a far cry from the Detroit Diesel engines would be an understatement. GM finally had something that could properly compete with what Ford and Dodge were offering.
The turbocharging system on the LB7 is unique compared to all other Duramax engines in the fact that it uses a fixed geometry turbocharger. As you’ll see later in the article, GM moved to a variable geometry turbocharger and never really looked back.
Really, the LB7 Duramax was quite a bit better than its rivals right out of the gate. With 235hp and 500 lb-ft of torque on tap, it was directly comparable to the 7.3L Powerstroke’s 235hp and 500 lb-ft and the 5.9L Cummins’ 235hp and 460 lb-ft figures. Once GM had upgraded that to 300hp and 520lb-ft, the LB7 was king on paper. Out in the real world, many LB7 owners were reporting fuel economy numbers as high as 22mpg, and a lot of that is thanks to the common-rail injection system.
Over time, and aside from the infamous fuel injector issues, the LB7 proved to be one of the most reliable diesel engines ever produced. GM’s first Duramax also provided a sound platform from which the General could build. It featured induction-hardened cylinder bores, a beefy forged steel crankshaft, cracked-cap rods, super lightweight pistons, and a bunch of other great features.
It was made in America, offered more power and torque than the competition, used lightweight cylinder heads and an ultra-lightweight rotating assembly, featured no EGR system, and it was backed up with two fantastic transmission options. It wasn’t free of problems, though.
Most notably, the OEM injectors are known for failing quite a lot, and it’s a labor-intensive repair, there’s no factory lift pump, the connecting rods cant hold much more than 550whp, locating dowels can shear, and the water pump is known for being quite problematic as power levels, and RPM get higher.
Of course, all good things have to come to an end, and midway through 2004, GM released the LLY Duramax.
The LLY Duramax
Realistically, the LLY Duramax is incredibly similar to the LB7 Duramax, and for a good reason. GM and Isuzu knocked it out of the park on the LB7, but since it had a few small issues, the LLY was released to get all of those fixed. Most notably, the LLY features a valvetrain that’s easier to work on, a new variable geometry turbo for improved throttle response and more low-end power, and the addition of an EGR system.
We’ve talked in-depth about EGR systems in other articles, but to quickly recap: EGR systems work by feeding exhaust gases back into the intake, to be reburned by the engine, because not everything is burned with just one engine cycle, so to further improve efficiency and use the unburned fuel, it is fed back into the intake system. GM really didn’t have much of choice but to add this to the LLY because of emissions regulations.
It’s particularly interesting that the LLY’s EGR system is basically problem-free, especially considering Ford’s 6.0L Powerstroke had some major issues with the EGR system. While it’s not as popular to delete the EGR on the LLY as compared to something like the 6.0L Powerstroke, it’s still something that many LLY owners choose to delete.
While the addition of EGR is a slight net negative on reliability, on the bright side, that new variable geometry turbocharger had a nice impact on power, which was increased to 310hp and 605lb-ft of torque. Interestingly enough, the turbo on the LLY is actually the largest turbo used on any Duramax engine. The VGT works by using movable vanes that direct exhaust flows across the turbine wheel to optimize performance.
Simply put, the VGT makes more power at any given RPM compared to the LB7’s fixed geometry turbo. This translates to an easy 500whp in any LLY truck, with just some simple bolt-on parts and tuning.
While GM designed the LLY as basically an upgraded and more reliable LB7, that didn’t exactly work out, as they introduced some new problems.
Most notably, the LLY is well known for overheating, which is a multi-source problem brought on by a handful of components. With the overheating, other components fail, and in really bad cases, you’ll blow a head gasket. Strangely enough, GM still refused to implement a factory lift pump with the LLY, but they did manage to fix all the injector issues that plagued the LB7.
Because of the total lack of problems on the LB7, GM kept the CP3 injection pump unchanged between the two engines. After all, Bosch designed the CP3 injection pump for engines much larger than the Duramax and that were expected to last well beyond 500,000 miles.
Internally, they kept the connecting rods the same between the LB7 and LLY since there was really no good reason to change them.
This brings us to 2006, where GM and Isuzu really stepped up their game and took the Duramax to a whole new level with the LBZ. Of course, at the time, diesel enthusiasts didn’t realize the LBZ would go on to become such a legendary engine, and really the last Duramax engine that was free of all the terrible modern emissions components we all love to hate.
The LBZ Duramax
Really, when the LBZ came around, it wasn’t exactly needed, in the sense that the LB7’s problems had been solved with the LLY, and most of the LLY’s issues were solved towards the end of its life. The Duramax engine program was very well sorted out, and there were almost zero major issues in terms of reliability.
That being said, Ford and Dodge weren’t going anywhere, so GM needed to drop something with a bump in power, and they did exactly that.
Right off the bat, the LBZ produced 360hp and 650lb-ft of torque, which was more than the 6.0L Powerstroke and the 5.9L Cummins offered.
The big things that made the LBZ such an upgrade over the LLY are the much-improved block casting, stronger connecting rods, higher pressure common-rail fuel system, and updated ECU. Honestly, you could make an argument that the LBZ is the best, most drivable, most reliable, and most tunable Duramax that GM ever produced, but we’ll see how that stands as we move towards the newer Duramax engines.
Although GM had slightly redesigned the LBZ’s connecting rods, mostly adding more material to the lower section, they didn’t quite upgrade the pistons in the same fashion. In fact, the pistons are actually the weak point on the LBZ, where they were a strong point on the LB7 and LLY, but more on that in just a second.
The connecting rods are forged steel and made with a similar cracked-cap design as previous Duramax connecting rods. The added material ultimately means a heavier rotating assembly, but it also means more power without a rod flying out the side of your block. The general rule of thumb is about 100hp or 200lb-ft. That’s how much more power the LBZ rods can stand up to as compared to the LB7 and LLY rods.
The pistons, on the other hand, are known for cracking in high-power applications. Almost all failed LBZ pistons crack along the centerline of the wrist pin. Speaking of which, the design of the wrist pin is something that many LBZ enthusiasts say contributes to the cracked pistons. More specifically, the LBZ’s use of wrist pin bushings is thought to be a contributor to the piston failure, but also the thinner wrist pins.
The LB7 and LLY pistons didn’t use wrist pin bushings for reference.
Best of all, GM knew the LBZ would make more power than the outgoing LLY, so they cast more webbing into the main bearing area of the LBZ’s block. They also used taller main bearing caps and deeper holes for the main bearing bolts. To put it simply, the LBZ block is beefy, and you’d be hard-pressed to get one to break unless you’re making a pretty insane amount of power.
Interestingly enough, the same Garrett GT3788VA turbo was used moving from the LLY. This definitely isn’t a bad thing, considering how capable it is, but it does suffer from some of the same sticking issues that the LLY also dealt with. Luckily, it’s nothing too bad, but just something to note.
It’s also worth noting that the LBZ was the first Duramax to be coupled to the six-speed Allison 1000 automatic transmission, which was a step up from the outgoing five-speed transmission before it.
As GM turned the power up, they also needed to turn the fueling up to keep up with the demand. They did this by turning up the injection system power from 23000 psi to 26000 psi, and once again they use the Bosch CP3 injection pump. To accompany the new higher-pressure CP3 pump, GM also added new fuel rails and injectors. The injectors were still supplied by Bosch, and they feature a revised seven-hole nozzle.
And best of all, the LBZ wasn’t hit with any new emissions components, which is also why GM ended up replacing the LBZ with the LMM in 2007. I would like to quickly note that although the LBZ is considered to be a “pre-emissions” engine, it still does have EGR and a catalyst in the exhaust.
The LMM Duramax
Depending on who you ask, some diesel enthusiasts will tell you the LMM is when the Duramax started to go downhill. Really, you could argue that all diesel engines started heading in the wrong direction around 2007 thanks to the emissions components manufacturers were forced to add.
Internally, the LMM Duramax is almost identical to the LBZ. In fact, it’s similar to the LBZ all around, but the biggest change comes in the form of a diesel particulate filter, or DPF. Compared to the LBZ, the LMM only had power bumped up 5hp and 10lb-ft. Really, the LMM is basically an LBZ with some added emissions components.
We’ve covered diesel particulate filters in other articles, but to briefly recap, the job of the DPF is simple: capture excessive exhaust soot and then burn it off occasionally through a regeneration mode. It’s also worth noting that other diesel engines, such as the Powerstroke, also feature a DPF. In fact, the DPF is the addition that made the 6.4L Powerstroke a piece of junk, but we’ve already made an article on that whole topic.
The LMM Duramax accomplishes the regeneration procedure by using the late injection method, which is basically injecting diesel fuel into the cylinders during the exhaust stroke as a way to get unburned diesel fuel into the exhaust system. While this method works, it also leads to the cylinder walls being stripped of lubrication, which then leads to excessive wear, low compression, and fuel diluting the engine oil.
In addition to meeting the DPF system, the LMM also has more exhaust gas recirculation than the Duramax engines before it. Ultimately, this called for a larger EGR cooler, and the square style EGR cooler used on the LMM is fairly robust compared to what you’ll find on other diesel engines. Even with that, though, it’s still known for clogging up with time. The EGR coolers are also known to crack and leak from time to time.
Outside of the annoying emissions systems that kill reliability, GM did upgrade the LMM in a few ways. Most notably, the LMM has improved heads that were superior to any Duramax heads up until that point. Compared to LBZ heads, the major difference is the coolant passages. Really, the coolant passages are the only major difference at all. GM knew the emissions components were going to heat things up quite a bit, so having the ability to keep everything cool was a big focus for GM’s engineers.
It also had revised injectors, which helped add power and offer a more complete combustion, ultimately reducing emissions output.
Unfortunately, the LMM uses the same pistons as the LBZ, but now with more power to deal with, plus the DPF and increased EGRs, making things even hotter. Ultimately, this means you’re more likely to see a piston fail on the LMM as compared to the LBZ. If you’re going to majorly turn up the power on your LMM, the pistons are going to be the biggest failing point, so definitely be sure to swap those out with some aftermarket ones.
Once again, though, emissions regulations were getting stricter, and GM was forced to make some changes, so in 2011 they introduced the LML Duramax.
The LML Duramax
Just like every Duramax rendition before it, the LML was heavily based on the engine which came before it. The majority of the big changes came in the form of added emissions components, which also had a negative effect on reliability. Luckily though, GM and Isuzu fixed some of the issues found on the LMM. Most notably, they changed how the active regeneration mode worked, but we’ll get to that in just a minute.
To start, let’s look at all the improvements GM made with the LML, starting with the cylinder block. The biggest changes to the block increased as improved casting and revised main bearing design. Like all previous Duramax engines, the blocks were cast in GM’s Defiance, Ohio plant but machined and finished 120 miles south at a different facility.
Aside from the block, GM also upgraded the oil pump. They knew the LML was going to deal with more heat thanks to the increased power output and the added emissions systems, so they upgraded the pump, drive gear, windage tray, oil pickup, and more. Most notably, this larger oil pump flows 11% more volume than the oil pump used on previous Duramax engines.
GM also realized the LBZ’s and LMM’s pistons were going to hold up to the job on the LML, so they removed the wrist pin bushings and improved the ends of the wrist pins for better piston support. Interestingly enough, GM managed to improve the strength of the pistons and decrease the weight.
Along with the new pistons came new connecting rods, but they’re very similar in terms of design and strength compared to the LBZ and LMM connecting rods. With some changes, they were able to reduce the weight of the connecting rods, as reducing rotating assembly weight was a big focus for GM’s engineers.
With the improved pistons, improved rods, and improved block, the LML is well known for holding up to 700whp without an issue.
Along with the improved block and rotating assembly, GM also improved the already very good injectors with even better injectors. Like before, the injectors came from Bosch, but they were designed to handle higher injection pressure and better fuel atomization in-cylinder this time around. It’s also worth noting that these piezoelectric allowed for two pilot injection events prior to the main event, which helped to keep the injection system ultra-quiet.
To go along with the new piezoelectric injectors was a new injection pump to replace the CP3 injection pump. Unfortunately, this was a pretty big downgrade. The injection pressure was turned up from 27000psi to 30000psi to support the new injectors and increased power output. However, the CP4.2 flows approximately 20-percent less fuel volume than the CP3 injection pump.
On top of that, the CP4.2 is much more sensitive to fuel contaminants or lubrication changes. To put it simply, the CP3 injection pump was much better in terms of total flow and reliability. But that brings us to the 9th injector that’s mounted on the downpipe. GM added this injector as a way solution for the exhaust injection type of system they were using before for active regeneration.
With this solution, the 9th injector receives its fuel from the CP4.2 pump and just injects it directly into the exhaust. By switching to this type of active regeneration system, the LML doesn’t have the oil dilution issues that the LMM dealt with.
But unfortunately, the emissions changes don’t stop there. Unfortunately, GM was pretty much forced to add a diesel exhaust fluid system to the LML in order to meet the stricter emissions standards. By injecting DEF into the exhaust after the cat and before the DPF during the regeneration cycle, harmful hydrocarbons are converted into carbon dioxide, nitrogen, and water.
By using this system, GM was basically able to reduce emissions after the exhaust left the engine, which allowed them to keep their EGR system the same. Ultimately, this meant more heat could be used in-cylinder to cut down on particulate matter, which culminates in less fuel being required during regeneration intervals. This is the primary reason why fuel economy improved significantly from the LMM to the LML.
The last major change GM made with the LML was a change to the turbocharger. This new turbo was a variant of the variable geometry turbo that GM used since the LLY. But, this version featured a smaller and more restrictive turbine side, which allowed GM to choke off the exhaust flow and use the turbo as a brake assist, also known as an exhaust brake. Technically it’s a turbo brake, but the point still stands.
This brings us to the L5P, which is the current Duramax engine that GM has been offering since 2017. All in, the L5P is still based on the Duramax engines before it, but this time around, it features more major design and part changes than any other Duramax
The L5P Duramax
Starting with the power output, the L5P is by far the most powerful Duramax engine, as you might’ve expected. It outputs a whopping 445hp and 910lb-ft of torque. Not only that, but it features some massive upgrades to support this power output. Most notably, the strongest Duramax crankcase yet. On top of that, this is the first Duramax engine to feature a lift pump, but we’ll get to the fueling upgrades in just a second.
The biggest carryover from the LML to the L5P is the bore and stroke in terms of the block. Past that, there a ton of major changes. According to GM, the new block is around 20% stronger, thanks to a new gray iron casting combined with heat treating. All in, the new casting and slightly taller deck height make this crankcase about 10lbs heavier than the previous one, but that’s a minor difference considering the 20% improvement on strength.
On top of that new stronger block are new cylinder heads. While they’re not a full redesign, the L5P heads are quite a bit different than the heads on the LML. A big cross-sections lead to shorter port distance from the intake valves and drastically revised intake ports bring not only more air, but equal airflow into each cylinder better than previous heads. The casting also got stronger, especially in the combustion chamber of the heads.
Inside the engine, things get even better, with stronger pistons and connecting rods. The rods are now constructed from powdered-metal, and while they still use a fractured cap design, it’s at a new 45-degree angle and features larger rod ends with bigger bearings.
Paired with those rods are new pistons, which feature a new fuel bowl designed. The pistons are taller and feature thicker walls than previous Duramax engines, and the cooling jets have around double the flow as compared to the LML, which simply means improved cooling.
One of the more interesting changes that GM made was ditching Garrett for the turbocharger and moving to BorgWarner. This new variable geometry turbocharger features an 11-blade billet compressor wheel with a 61mm inducer. For reference, the LML’s turbo featured a 60.6mm cast inducer wheel. This new turbo is one of the main components that allow the L5P to make so much power in stock form, but also a ton of power with some tuning.
They didn’t just stop there because they also decided to ditch Bosch for their fueling system. More specifically, the L5P features a Denso HP4 injection pump to replace the Bosch CP4.2 pump. This new pump features more high-pressure cylinders, an integrated filter screen, and an electric in-tank lift pump, which is another first for a Duramax engine. Put all this together, and you’re looking for an injection pump capable of producing over 36000psi without an issue.
With that new Denso injection pump also came new Denso injectors. Strangely enough, they ditched the piezoelectric injectors in the LML and went back to solenoid-style injectors, but these new Denso injectors flow around 20% more fuel than the LML injectors. Combine the increased flow with more injection events per combustion cycle, and you have the best injectors to be featured on a Duramax.
All of these changes add up to arguably the best Duramax engine yet. Now, I know a lot of people out there will argue the LBZ is better, and while it’s a valid argument, it’s not something we’ll be getting into with this article. With some aftermarket parts and tuning, you can easily see a stock turbo, stock bottom end L5P Duramax make over 600whp. For reference, any previous Duramax engine will really struggle to get past 500whp without an upgraded injection system and turbo.