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If you stop and think about it, the word “diesel” conjures up a lot of things – durability, torque, and towing power, just to name a few. We think of diesel engines as some of the most robust and reliable machines in existence, and for good reason; they power everything from cargo ships on the open seas to the three-quarter-tons rolling down American highways.

By and large, no matter if you drive a Power Stroke, Duramax, or Cummins-powered truck, you drive a capable vehicle with an engine built to withstand extremes that would leave gas motors smoking by the side of the road. Still, these diesel powerplants are not perfect, and we thought it would be good to explore the biggest flaws involved in each brand.

For this article, we will look at the top issues affecting the 6.7-liter Power Stroke, the 6.6-liter LML Duramax, and the 6.7-liter ISBe Cummins.

To that end, we spoke with diesel experts from three different businesses – American Diesel in Murrieta, California; Strictly Diesel in Phoenix, Arizona; and Industrial Injection Diesel Performance in Salt Lake City, Utah – to learn how the mainstays of each American brand suffered failures throughout the years.

6.7-liter Power Stroke: DualBoost

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The Honeywell-built GT-32 DualBoost turbocharger was a bugbear on the 2011-14 model years of the 6.7-liter Power Stroke.

Ford’s 6.7-liter Power Stroke was a big departure from standard operating procedure when it first came out. Rather than continuing the line of International-built motors that started way back with the 7.3-liter in the 1990s, the Blue Oval went completely for an in-house design.

One of the things that separated the 6.7-liter from the rest of the pack was its Honeywell-built GT32 turbocharger, which used a piece of technology called DualBoost. “It used double-sided vanes on the compressor and impeller wheels, hence the name,” explained Strictly Diesel’s Gary Maschner. “By contrast, most manufacturers use single-sided vanes. The point of the DualBoost was to facilitate faster spooling, offering greater throttle response than before.”

As innovative as it may seem, the DualBoost was also its own worst enemy. Tuning the engine to increase horsepower and torque would eventually cause the turbocharger to eat away at its ceramic ball bearings, which would lead to excessive play in the compressors. “You could tell it was going bad when you heard a loud and screeching noise,” said Maschner.

This problem affected Super Duties all the way until the 2015 model year, when the turbocharger was redesigned and given a more commonplace compressor setup. It now runs on the GT37 single variable geometry turbocharger, which hasn’t had any significant problems as of yet.

6.6-liter Duramax: CP4 Fuel Pump

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The Duramax was jointly developed by GM and Isuzu in the early 2000s to get away from the Detroit Diesel days of yore. Miraculously, the engine has had incredible staying power in an ever-changing arena of ideas regarding emissions, efficiency, and power numbers. Proof of this comes from the fact that it has never dipped or risen above the 6.6 liters of air displacement, despite going through six iterations – LB7, LLY, LBZ, LMM, LML, and now the L5P.

The L5P hasn’t yet been put through its paces to see what lies beneath, but the LML has. Releases in 2011, the engine brought with it the “ninth injector” that sprayed fuel on the DPF during regeneration cycles, as well as upgrades to the engine block’s strength, the valvetrain, and the oiling system.

On the Duramax Diesels forum, user aarolar discovered his CP4 had bitten the dust after Independence Day 2016. He decided to replace it with a CP3K kit from Fleece Performance. Photos: duramaxdiesels.com

However, the Achilles heel of the LML is its CP4 fuel pump. More to the point, the blame seems to lie with the diesel fuel that has been made standard in North American gas stations since 2006 – USLD, or Ultra-Low-Sulfur Diesel. While the fuel is optimal for the sake of reducing harmful emissions, it has the deleterious effect of having less lubricity than the diesel fuel that came before. Less lubricity makes it difficult on the CP4’s hydraulic pistons to continue working, and consequently, the pistons seize up. This leads to either bricking the pump or an explosion that causes metal shavings to be dispersed throughout the entire fueling system.

“The tappet on the plunger rides on the cam, and when there’s a lack of lubricity there, the tappet will seize,” said Cody Williams of Industrial Injection. “The tappet will lift, and the roller won’t be square on the cam profile. The tappet can’t do its job anymore, and the pump eventually seizes or blows up.”

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One solution to the CP4 is to simply buy a CP3, which is arguably better than its successor in that it delivers fuel with less stress to the internals. Photo: Industrial Injection

Granted, from what we could shake out from internet forums, the chances of this happening were quite low; that is to say, it is a rare problem. Still, the heavy cost of replacing the fueling system makes it one of the biggest concerns to think about when considering a late-model Duramax.

There are two solutions to this problem, the first of which is to buy a lift pump. “Buying a lift pump will provide more fuel to the pump, which supplies fuel to the low-pressure side of the CP4,” said Williams. “That will generate more lubricity to the pump.”

The next option is more popular and involves swapping out the CP4 for its predecessor, the CP3. “The CP3 didn’t have the same issue that the CP4 had, so it’s a better design in terms of longevity,” said Williams.

6.7-liter Cummins: Exhaust Manifolds

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Cummins and Ram have been joined at the hip since 1989, when the first 6BT-powered pickup rolled into showrooms, impressing truck drivers with the platform’s simplicity, power, and durability. 27 years later, that relationship is still going strong, and the latest engine has been the 6.7-liter ISBe.

Debuting in 2007, the 6.7 has carried the torch of its predecessors, putting trusted reliability and output as its main strengths. Where the engine trips up is not on the intake or fueling side of things, but rather the exhaust.

Over time, the 6.7-liter Cummins' exhaust manifold can shrink from excessive heat. From that, it can lead to cracking and lost efficiency in the exhaust system. A good solution is get an aftermarket manifold, like we did on Project Diesel Chase with BD Diesel Performance.

“The manifold gasket can be an issue,” said Maschner. “That’s because the manifolds shrink from excessive heat, even on stock trucks. I’ve seen ones where they’ve shrunken by at least a quarter of an inch. When this happens, the mounting bolts of the manifolds break.”

Maschner believes it has to do with towing and how much a truck does of it, but outside of that, the manifolds simply can’t take the heat, as it were. “The aftermarket options out there are an improvement,” he said. “The good manifolds or headers have webbing in between the ports, which helps keep the structural integrity from being compromised.”

General Problems: Turbocharger Cool-Down And SCR

Photo: Honeywell

Now that we’ve looked at a top issue for all three of the major diesel engines, now we can examine something that affects all three: turbochargers and DEF/SCR.

Speaking with Quince Graveen of American Diesel, the problem that comes up time and again, no matter the brand, involves cooling the turbocharger.

“The big thing for any of these turbodiesels is to idle down before you shut off the motor,” he said. “This is to cool the turbocharger down. When you take a turbodiesel out on the highway, you can get it up to 1,000 degrees Fahrenheit. Taking it back home and shutting it right off denies the turbocharger the opportunity to cycle oil through and cool down. So, make sure you let the truck idle for a few minutes, and your turbocharger will last a good while longer.”

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Hate seeing this on your dashboard? Then let your truck complete its regeneration cycle if and when it comes. Photo: duramaxforum.com

Maschner’s advice was to always make sure the truck can complete a regeneration cycle on the DPF. “Not letting the regen cycle complete will cut down the life of the DPF, and your truck will throw a code a lot sooner than it ought to,” he said. “Also, make sure the quality of DEF you use is up to snuff.”

What do you find to be some of the biggest issues affecting the current crop of diesel trucks? Have you learned of any problems affecting the Cummins 5.0-liter or VM Motori 3.0-liter EcoDiesel? Let us know your thoughts in the comments section below.