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What Breaks When: 2011-2016 Chevy Duramax LML

Diesel Performance Parts, Diesel Truck Maintenance, GM Duramax, Products, Tips and Tricks

Written By: Lawrence “LT” Tolman

2011 was a big year for GM as there were a lot of major changes made to its HD truck line-up, but the crazy thing was, from the outside you really couldn’t tell. Most of the major body parts on the Silverado and Sierra were un-changed and only minor details would set a 2010 apart from a 2011. The visual changes included a slightly restyled front bumper and grille and slight tweaks to update the interior, but for the most part the visual appearance of the GMT900 platform was the same as it had been since they were released halfway through 2007. However, once you looked a bit deeper, it became readily apparent the two trucks were nothing alike. Just about every mechanical system of the truck had been updated to make the new HD’s stronger, burn cleaner, more powerful and able to tow heavier payloads. The re-design included an all-new fully boxed chassis to replace the earlier C-channel frame which had been around since 2001 and major revisions were made to the steering and suspension. The trucks sported a new larger wheel bolt-pattern of 8x180mm, the Allison transmission got several updates to make it hold more power and shift crisper and of course you can’t forget about the all new powerplant, the LML Duramax.

Details of the LML

By 2011, the Duramax lineup had already seen four major revisions, starting out with the LB7 in ‘01, moving to the LLY in ’04½, the LBZ in ‘06 and the LMM in ‘07½. In 2011, federal emissions regulations had tightened, so additional measures had to be taken to keep the Duramax compliant, so the LML was the first to be equipped with Selective Catalyst Reduction (or SCR), a system which used a urea-based exhaust fluid to break down harmful nitrogen oxides. Also included in the emissions arsenal was a 9th injector, or dosing valve, which sprayed fuel directly into the exhaust stream during a re-gen to clean out the exhaust filter. This eliminated the problem LMM’s had where diesel fuel would wind up in the engine oil, which could lead to major problems. So, by relying on an external fuel source for a DPF re-gen, the LML kept its oil free of any fuel dilution. Speaking of fuel, the LML was the first Duramax to be able to run on a B20 blend of biodiesel.

With its advertised output of 397hp and 765 pounds of torque, the LML was the most powerful Duramax to date. While there were several other mechanical upgrades to the internals of the engine like stronger pistons and rods, the biggest difference between the LML and all earlier engines was in the fuel system, which now centered around a Bosch CP4.2 injection pump and new piezoelectric injectors which operated at a much higher pressure than earlier models, topping out near 29,000psi. This higher injection pressure meant the fuel was atomized into much finer droplets, which resulted in more efficient combustion, formation of less harmful pollutants and everyone’s favorite, more power. That all sounds great on paper, but in the real world, things weren’t so simple. In fact, this new injection pump was the cause of quite a big headache.

The Bosch CP4.2 in all its glory, the next generation of injection pumps. Will it stick around? Who knows.

CP4 Problems

The newest Bosch pump was very different from the earlier CP3; it features two piston pumping elements instead of one and each has its own camshaft lobe; it’s basically a V-twin injection pump. In order to produce such high pressure inside the fuel rail, the internal parts of the CP4 are machined to a very precise and tight tolerance and diesel fuel is the only lubricant the moving parts have, which is the root of the problem. Because of the physical properties of diesel fuel, it loves to collect water and other contaminants, neither of which are very good at lubricating metal.

If your fuel system hasn’t been maintained with regular filter changes or you get a bad batch of fuel, there can be a small amount of trash or debris that winds up in the CP4 pump, which can be just as dangerous as Luke Skywalker is to the Death Star. If they make it past the filter and into the pump, those tiny dirt or water droplets can actually act as an abrasive and cause small amounts of metal to wear away. As soon as one aluminum chip or shaving is dislodged, an irreversible chain reaction starts, which causes more and more wear to take place, which generates more and more debris, which in turn causes more wear and the cycle keeps getting worse until the injection pump won’t develop fuel pressure and the truck simply won’t run anymore. That sounds like an inconvenience, but those metal shavings aren’t just recirculating in the injection pump. When a CP4 lets go, it sends metal shavings directly into the main artery of the fuel system and first stop is the fuel rail, followed by the injectors, the return lines and ultimately, they’ll make their way back to the tank. In other words, your entire fuel system will be filled with metal shavings, your injectors will be toast and you aren’t going anywhere. The worst part about a CP4 failure is how little (if any) warning you get. In most cases, you will be driving around when suddenly you might notice a slight loss in performance. Sometimes you’ll get a “change fuel filter” message triggered by low rail pressure and other times you will get a check engine light with a DTC code relating to the fuel system (P0087, P0088, P0191, or P128E). The problem is, once the shrapnel gets into the fuel system and the chain reaction starts, you might only make it a couple more miles until the whole thing implodes and the truck is inoperable.

If you find yourself in this situation without a warranty, you may want to sit down for this next bit of news. GM recommends the following procedure in the event of a failed CP4: replace the injection pump, fuel rails, all eight injectors, all high-pressure lines, low pressure injector return lines and the fuel filter. On top of that, you’ll also need to clean out the main feed and return lines running the length of the chassis, all engine mounted rubber fuel lines, the fuel tank itself and even the sending unit. If paying out of pocket at a dealer, the total cost of parts and labor can exceed $10,000. The only consolation a Duramax guy might have as he reads his invoice, is in addition to the LML, the 6.7-liter Powerstroke is also powered by the Bosch CP4 injection pump and it too has a reputation of grenading itself.

Ain’t Nobody Got Time for That

It’s time for some real talk: the purpose of this article is not to scare you into selling your CP4 powered truck or to stop you from buying one in the first place. While the failure is a well-known issue, it seems the occurrences are hit or miss, sometimes even sporadic. With a little digging, it’s clear there have been some CP4 failures with as little as 30,000 miles on the clock, but at the other end of the spectrum, some have logged over 700,000 miles on their LML with the original injection pump still running strong. For most owners who have experienced the issue, it seems the average failure happens just under 100k and luckily GM’s warranty department picks up the tab for the repair. It’s also worth noting in 2018 a class-action lawsuit was filed against GM, Ford and Bosch for the early failures of the CP4 pump, however, it appears the case was dismissed for a bunch of legal reasons containing the phrase “denied as moot”. In my opinion, the CP4 issue is a bit blown out of proportion. I’ve spent a lot of time trying to find information on how many failures have occurred and it’s hard to get a solid number. The results of a few online polls and some simple extrapolation indicate less than 10% of all LML’s have had a failure and other reports suggest the figure is around 5%. Either way, the odds aren’t that bad. It seems the biggest issue isn’t how many pumps fail, but rather the cost to make the repairs after the failure happens. Even if you go to an independent repair shop and find the best possible online price for replacement parts, the cost of getting back on the road is a tough pill to swallow, when the repair can represent a huge percentage of the trucks actual value.

A CP3 conversion kit is as simple as this. Some kits will require custom tuning, but beyond that, its all bolt-on and you’ll gain a substantial amount of dependability.

CP4 Fixes

Now that we’ve got the dirty laundry all aired out, it’s time to talk about how to prevent and repair the problem. It’s been proven the number one cause of a failing CP4 is the fuel that’s going through it so clearly it’s important to keep said fuel clean. I’ve mentioned this before, so it feels like I’m beating a dead horse, but regular maintenance is absolutely necessary on any diesel engine. As such, it continues to surprise me how many people forget to replace the fuel filter. I know it’s convenient GM builds in a fuel filter reminder which pops up on the dash when the computer calculates your filter is dirty, but rather than waiting for a computer to tell you, why not just go ahead and change it every 10,000 miles like a normal person? You’re never going to damage your engine by changing the fuel filter too often, but not changing it enough definitely has its risks. A good choice for a stock fuel filter replacement on the Duramax is the AFE Pro-Guard, which removes 99% of water and traps dirt particles 4 microns and larger.

While removing dirt and contaminants from your fuel is a good start, it doesn’t do anything to improve the lubrication properties of fuel in the first place. An old wife’s tale says you can just dump some ATF into the fuel tank, but don’t do that. Instead, use a proper fuel additive like F-Bomb. For the LML, the most important effect will be increased lubricity of the fuel (to help the CP4), but you’ll also get a bump in cetane, an improvement in fuel economy and for guys who park their truck more than they use them, it will prevent bacteria and algae from growing in your fuel (yes, algae like in a fish tank. It’s a real thing…)

One more way you can help extend the life of your CP4 is take away some of its stress. The CP4 is responsible for drawing fuel from the tank as well as pressurizing it, so by adding a lift pump and sharing some of the workload, its stress is greatly reduced, and the injection pump can work more efficiently. Taking a lift pump one step further and adding some additional filtration and air separation into the mix is the icing on the cake. The FASS Titanium Signature Series lift pump will remove dirt, water, air and debris from your fuel and sends 8-10 psi of fuel up to the engine. The 165 gallon per hour unit is the most popular and is ideal for engines making between six to nine hundred horsepower, but because of the way the FASS pumps are regulated, it will work perfect at much lower power levels and daily driving as well. If you need more fuel capacity than that, FASS also offers pumps that flow 250 and 290 gallons per hour, which will support up to 1,500 horsepower from a single pump. The best part is the high-efficiency filters only require changing every 30,00 miles.

Upgrading any diesel truck, especially an LML Duramax, with a FASS lift pump is one of the best investments you can make.

One last trick to keep the CP4 from wiping out the rest of the fuel system is to swap out the inlet metering valve on the pump with a System Saver Inlet Valve. While it won’t stop a CP4 from failing and generating debris, what it will do is prevent the debris from wiping out the injectors and contaminating the rest of the system. They are very affordable compared to a set of injectors, and easy to install as well, so a worthwhile insurance policy by any standard. If despite all your best efforts you still have a CP4 wipeout, you’ll have to do some shopping. When it comes to injectors, your best bet is to go with a genuine Bosch OE replacement. They are made with all new construction, so there’s no need to send your cores back. Since you need to replace all eight after a pump failure, it might be worth your while to consider a larger than stock injector if you plan on boosting the power level. A 50hp set of injectors from Dynomite Diesel can add a little spunk to your LML since they flow 20% more fuel than stock and they’ll support around 650hp at the wheels while still being perfectly drivable and can also give a slight bump in fuel mileage.

As far as the injection pump is concerned, you’ll obviously need another, and you have some options as well. Of course, the easiest and most affordable option is to go with a stock replacement. Dynomite Diesel has you covered there as well and for just over 800 bucks you can have a direct replacement of the stock pump. One thing you might not know, is the CP4 has a built-in power disadvantage to the CP3 with its lower fuel volume, so one popular move is to swap it out with a CP3. A conversion kit from Industrial Injection makes the job simple, with all the lines and brackets you’ll need to install the earlier style pump. The benefits of a CP3 conversion are twofold: you’ll get a much more reliable pump right off the bat and more power potential as well. The basic kit can support 600 HP and if you want more than that, a larger displacement CP3 will swap right in, as well.

Chasing that High (Power Number)

Once the fuel system is taken care of, you’re pretty much out of the woods. Honestly, even with all the emissions components intact, the LML will be reliable for many years and miles to come. There are a few ways you can get into trouble however, but that’s usually a case of self-inflicted harm from being a bit too eager with the power upgrades. When compared to all the other stock Duramax engines, the LML has the strongest and lightest rotating assembly. The pistons are made from cast aluminum with a steel ring land and the design has been updated to make them stronger and less prone to cracking than the LBZ/LMM design. The LML connecting rods are made from forged steel, have roughly the same strength as earlier rods and are a few grams lighter. Even with all those improvements, the magic number for the power limit of a stock engine is somewhere around the 700-horsepower mark. Since that’s pretty easy to achieve with modern turbos and tuning, you’ll want to leave yourself some margin of error if you want your engine to last. But if you have the need for speed, just be sure to set aside a couple extra bucks, since you’ll inevitably be tearing out the engine for an upgrade sooner or later.

In order to keep maximum drivability, turbine braking and throttle response, the stock turbocharger on the LML is the smallest of any VGT Duramax. The vanes in the exhaust housing measure in at just over 11mm thick (down from 15), it has a 60.6mm inducer on the compressor wheel and a 62/70mm turbine wheel and the turbocharger will support 520-550 rear wheel horsepower. However, if you push your turbo to the edge, the exhaust drive pressure (and temperature) goes up tremendously since the turbine housing is such a major flow restriction. This is not a good thing if you want your internal engine parts to last and is especially problematic if you are thinking about compound charging your LML. Instead, you’ll first want to consider a turbo with a much less restrictive exhaust housing and taller vanes, like the Duramax Tuner Stealth 64. This would be a great upgrade from stock in terms of horsepower and reliability and the 64mm charger will still drive like a stock turbo. All this means towing heavy loads is still on the table, you’ll have great EGT control and the Stealth 64 will support 620 horsepower when used in a single application. If that’s still not enough, you can step up to the Stealth 67G2 which has all the same benefits as its smaller brother, but the big boy can lay down 800 horses.

Discrete, yet big on the Richter Scale. The drop-in Stealth 64 Turbo from Duramax Tuner delivers a big punch in performance and is simple to install.

What the Hell, Buy an LML?

So, the question becomes, is it worth owning an LML? In short, I believe the answer is yes. Over the five year span the engines were produced, there were two body styles of trucks that were equipped with the engine, the ride quality of which was greatly improved over earlier models (along with the towing capacity) and the interiors have plenty of creature comforts. Sure, you might have this constant fear lurking over you about the dreaded CP4 failure, but don’t let it get you down. The actual odds of a failure are much less than the internet would have you believe and with proper maintenance and a few simple preventative measures you’ll likely never experience a failure. If you still are worried, the best bet is simply to swap out the CP4 before it fails in favor of a CP3 and you’ll eliminate the problem altogether. The best part of doing that, is since your injectors won’t need replacing, you’ll have money left over in the budget for a set of compounds and a built trans.


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