Not so long ago, diesel engines were considered as something that should not be placed under the hood of a car. With tractor-like sound, vibrations and the high amounts of black smoke from the exhaust, they were just not refined enough. Today, with modern diesel engines, it is a whole different story. These machines have several strong points, with low fuel consumption being just one of them. The application of variable turbochargers and sophisticated fuel injection systems enabled wide torque range and faster throttle response, while offering a considerable engine noise and vibration reduction. And in the end, various exhaust treatment systems eliminated unpleasant smells and black smoke from the tailpipe. As a result, diesel powered vehicles offer strong pull from low revs combined with a smooth and civilized engine characteristic. All of this makes them ideal for pulling heavy loads or towing a trailer. So it is no surprise to find diesel engines as a desirable option in modern pickup trucks.
Still, don't think that these engines are without a flaw, as there are several downsides.
Although sophisticated fuel and induction systems improve overall driving experience, they are also less robust and can be more prone to breakdowns if neglected. High-quality fuel and regular maintenance have become an imperative, as even a small amount of low-end fuel could cause irreversible damage to the common rail system. Additionally, demands on oil quality have increased, as oil vapor and carbon build-ups can jam Variable geometry turbochargers.
If not driven carefully, massive amounts of torque can put an excessive strain to the transmission and drivetrain in general. This is especially evident in cars with manual transmission, where flywheels and clutches can fail.
Yet, the biggest problem is high amount of harmful pollutant in exhaust gases, which have promoted emission regulation as a primary cause for speedy engine development over the past
ten years. Every few years, new government regulations increasingly demands reduction harmful pollutant emissions. This forces vehicle manufacturers to install more and more complex devices and systems to meet those regulations. There are several ways to reduce emissions. Diesel Particle Filters (DPF), are used to eliminate soot, but they don't do anything about the nitrogen oxide emission. This is where either Selective Catalytic Reduction (SCR) or Exhaust Gas Recirculation (EGR) come into play.
Just like many other manufacturers, GM, Ford and Chrysler have their diesel engines that are installed in cars, SUVs and trucks. Not only that these state-of-the-art machines have similar engine displacements and power outputs, but they also utilize similar turbochargers, fuel injection and emission control systems. But don't think that there are no noticeable differences between three of them. In this article, we will make an in-depth analysis and comparison of these engines, showing how they are designed, what are their strong-points and weakness, as well as how they were developing over the years.
The Cummins ITB 6.7L engine
Out of three aforementioned engines, Cummins is by far the oldest brand. Although generally associated with Dodge pickup trucks, this engine is actually designed and manufactured by a specialized diesel power-plant manufacturer that bears that name. A current engine is the latest of the B-series engine family, introduced mid-model year in 2007. This is a 6.7L straight six unit, with a conventional gray cast iron engine block. Unlike other modern engines, the 4-valve-per-cylinder head is also made from cast iron. As OHV valvetrain is gear driven, there is no need for complicated timing chains or belts. A relatively low compression ratio enables the usage of a quite large Holset turbocharger, capable of producing 33 psi of boost. To round all up, Bosch common rail system ensures noise reduction and smooth operation. Up to 2013, emission control system utilizes DPF and EGR, making DEF fluid unnecessary. However, that changed in year 2013, when SCR system was also added. With weight of around 1.100 lb, it is a bit on the heavy side. It is available either with a Chrysler-built six-speed 68RFE automatic transmission or with a Mercedes G56 6-speed manual transmission.
During 2019, a revised ITB 6.7L engine is available in several heavy duty RAM versions. This high-output version has engine block made out of compacted graphite iron, which increases strength without adding weight, forged steel connecting rods and a revised cylinder head. Combined with all new Bosch CP4.2 common rail injection and a modified turbocharger, this new engine is the first of it's kind capable of reaching the 1,000 lb-ft barrier.
You can find these engines in various Dodge RAM versions.
Specification of ITB 6.7L Cummins
Common faults with the Cummins engine
Clogged Diesel Particulate Filter (DPF) is a very common problem on earlier 6.7L engines. Due to the fact that they did not use SCR system, these engines tend to operate with rich air-fuel mixture. This is done to keep the NOx down, but it also generates significantly more particulates and loads-up the DPF much quicker.
Variable Geometry Turbocharger (VTG) is a fairly common failure point. As the 6.7L engine was the first Cummins unit that uses variable geometry turbocharger, it is not unusual to have some initial problems. These units have a series of mechanisms that alter the turbine housing geometry. Problems occur when the moving components in the turbine housing become coated with soot and oil, as the excessive buildup will prevent the movement of the VGT mechanism. This generally leads to excessive turbo lag or poor top end engine power. Also, turbocharger unit tends to fail from various causes, ranging from leaking oil seals to broken turbine or compressor wheels.
Head gasket failures - unlike its predecessor 5.9L engines, the new 6.7L engine is known to have problems with failing head gaskets. This is caused by higher cylinder pressures which are required for achieving such high torque output from a relatively small engine.
Fuel dilution - the root cause for excessive fuel dilution lays within the method in which the engine manages regeneration. This is the process which burns and removes the particulate matter captured in the DPF. In 6.7L Cummins engine uses post-injection method releases raw fuel into a cylinder during the exhaust stroke after normal combustion has occurred. However, as this fuel sticks to the cylinder, small amounts eventually escape past the cylinder rings and enter the crankcase, contaminating the engine oil.
History of Cummins engines
Although the Cummins automotive diesel engines lineup goes way back to the beginning of the 20th century, we will focus on the units that were installed in the Dodge RAM.
6BT 5.9L 12V engine was first installed in RAM in the 1989, as an alternative to V8 gasoline engines. As this engine generated much more low-down torque with a significantly increased fuel economy, it quickly became a very popular option. Over the years, this engine received many updates on the fuel injection and turbocharging systems. This all cast iron engine had 2-valves-per-cylinder head and it was turbocharged. While first models had mechanical direct fuel injection and were not equipped with an intercooler, this was gradually changed. The intercooler was introduced in 1991 to help boost up the power, and in 1994 a new P7100 Bosch injection pump replaced the outdated system.
IBS 5.9L 24V replaced the well-proven 6BT engine in the 1998, to meet updated emissions requirements. It featured many improvements, with all new 4-valves-per-cylinder head and VP44 Bosch electronic fuel injection system being most notable. This resulted in increased power output and performance potential without compromising reliability and durability. Another mayor update was done in 2003, when the common-rail injection system was introduced. Not only that this additionally increased power output, but it also dramatically reduced engine noise and vibrations, all of which helped it being selected as one of Ward's "10 Best Engines" for the year. At the end, it is worth noticing that this engine was able to meet emission regulation without using DPF or EGR.
The Power Stroke 6.7L engine
The current 6.7L Power Stroke engine that comes in Super-Duty trucks series is Ford all in-house built diesel engine. Nicknamed the Scorpion, this is Ford's first venture into heavy duty diesel power-plant waters, as all previous Power Stroke units were produced by Navistar. With that in mind, it is only natural that potential buyers were worried about the reliability of this newly designed platform. Luckily, it turned out that Ford got everything right with the 6.7L. It offers great performance, fuel economy, and reliability. Maintenance-free timing is done by chan-driven OHV setup that operates a 4-valves-per-cylinder valvetrain. The engine is quite light, as the block is made from compacted graphite iron, mated to a aluminum heads, while V-8 engine layout ensures compactness. Powerstroke engines have a unique exhaust and intake setup. Unlike a traditional V-engine, the intake manifolds are located on the outer deck of the cylinder head and the exhaust manifolds exit directly into the engine valley where the turbocharger is mounted. This exhaust flow design increased thermal efficiency of the turbocharger. Another useful feature is a driver-activated engine-exhaust brake, which restricts the exhaust flow. This generates back pressure and slows the vehicle. On the emission control front, this engine utilizes almost every technology available. In addition to standard DOC, EGR and DPF setup, it also uses SCR to additionally lower NOx content. These engines come equipped with a 6R140 6-speed automatic transmission, called TorqShift by Ford. Over the course of time, it has proven to be one of the best factory-installed transmissions on the market.
These engines are available in 2011 to present Ford Super Duty Series (F-250/F-350/F-450), as well as in 2015 to present Ford F-650/F-750 medium duty trucks.
Specification of Power Stroke 6.7L engine
Common issues with the Power Stroke engine
Most problems with the 6.7L Power Stroke are isolated incidents - failures or faults experienced by a low percentage of owners, particularly on early engines. While issues are expected from a clean-late engine, especially one as advanced as the 6.7L Power Stroke, there have been very few common issues with Ford's new diesel platform.
Radiator failures and general coolant leaks are a somewhat common on the early 2011 trucks. These trucks have two radiators, and the one closer to the engine develops a leak on the spot where the plastic tank is jointed to the metal core. It is also possible to see coolant leaks around the turbocharger area, with coolant inlet fittings being a usual cause.
Turbo failure is one of the most common issues on these engines. As the turbocharger on these trucks is a fairly complicated unit, early 2011-2012 models all suffer from various turbo-related problems. The ceramic bearings are the most common failure point.
EGR System Issues are far less common when compared to earlier 6.0 and 6.4 Powerstroke engines. Still, the problem with EGR cooler getting clogged by soot still persists. Result is an obstructed exhaust flow and a 'check engine' light.
SCR system is a relatively simple device, but it still has a several weak points. A built in-tank heater element can burn out, or the injector and connecting tubing can clog up. As this system works only at high exhaust temperatures, urea solution can crystallize if you have frequent short journeys. Also, NOx sensor can break or give false readings.
Glow Plug Failure is a potentially the most dangerous weak point on 2011 trucks. The problem lays within the glow plugs, which can break off. In most cases, this causes catastrophic engine damage. Newer trucks have updated glow plugs and do not suffer this issue.
Fuel System Failure can be caused by an internal deterioration of the high-pressure fuel pump. When this happens, the debris are spread throughout the entire fuel system, requiring new pump, injectors, and pressure regulators. To prevent this, be extremely careful on what fuel you use, and change the filters regularly.
NOx sensor was a extremely common failure on 2011 trucks. A faulty NOx sensor can, in some cases, cause engine power reduction. Most of these sensors were replaced in first years of service.
The Duramax L5P 6.6L engine
Duramax, a GM's answer in growing heavy-duty diesel engine segment, entered the stage quite late when compared to Ford and Dodge. But even in its first version, released in 2001, it featured the most advanced solutions, such as common rail fuel injection or aluminum heads. Although it was speculated that the current L5P will be developed from the scratch, the general layout was left unchanged, keeping both displacement and an identical V8 arrangement. To keep up with the competition, this engine uses a cast iron block with hardened cylinder walls, mated with aluminum 4-valves-per-cylinder heads. Much like Ford counterpart, it uses chain driven OHV setup, with a camshaft in the block and pushrods. An electronically controlled variable geometry turbocharger in combination with a high-efficiency air-to-air intercooler, helps increasing the torque throughout the RPM range. And the newest generation Denso common rail fuel injection capable of achieving fuel pressures up to 29.000 psi brings down the fuel consumption and noticeably reduces engine noise. Starting in extreme cold is much easier with all new ceramic glow plugs, designed to reduce preheat times. Exhaust treatment such as EGR, DPF and SCR ensure that the strict emission regulations are not broken. This engine is only available with a well proven Allison 1000 6-speed transmission.
You will find these engines in 2017 to present Chevrolet Silverado and GMC Sierra HD pickup trucks.
Specification of Duramax 6.6L engine
Bore and Stroke
103 mm x 99 mm
Block Material
Grey iron w/ induction hardened cylinder walls
Head Material
Aluminum
Valvetrain
OHV with 4 valves per cylinder
Compression ratio
16.0:1
Turbocharger
Electronically controlled and actuated variable geometry turbocharger
Fuel system
Denso Common Rail High pressure Direct injection
Power output
445 hp @ 2.800 rpm
Torque output
910 lb⋅ft (1234 Nm) @ 1.600 rpm
Common issues with the Duramax engine
Fuel starvation usually caused by air in the fuel lines. The root of the problem is the fuel filter housing design and the absence of the fuel lift pump. Duramax engines use the high pressure fuel pump, which vacuums fuel from the tank. In many cases, air seeps in either through a small crack on the fuel filter housing, or through a bad housing o-ring. It can also be relatively difficult to prime the fuel system once air has been introduced into the system, such as when the fuel filter is changed.
Cooling issues on Duramax engines are quite common, with a faulty coolant pump taking the top spot. Problems seem to be caused by a factory design. As a preventive measure, coolant pump is best replaced in the 80,000 - 100,000 mile intervals. It is also possible to experience various overheating problems, caused either by a dirty radiator or a faulty fan clutch.
Injection system can cause several problems. In general, fuel injectors are quite sensitive to contamination and may fail prematurely if poor quality fuel is used. Also, a proper maintenance and regular fuel filter changes are very important. Other possible weak point is an injector harness that chafes over time. With wires becoming exposed, a number of issues can occur, including a no-start or rough running, lack of power situation.
Design of the positive crankcase ventilation (PCV) system proved to be a bit troublesome, as it can vents the crankcase pressure into the intake. With time, engine oil fumes build-up within the intercooler and connecting tubing. Not only that this clog up, but it can also cause intercooler tubing deterioration.
SCR system is a relatively simple device, but it still has several weak points. A built in-tank heater element can burn out, or the injector and connecting tubing can clog up. As this system works only at high exhaust temperatures, urea solution can crystallize if you have frequent short journeys. Also, NOx sensor can break or give false readings.
History of Duramax engines
No matter what year or generation, any Duramax is a 6.6L V8 unit. Unlike Cummins or Ford, who altered key engine features such as displacement, cylinder head design or fuel injection throughout the years, GM retained the same general layout from the start.
LB7 was the first Duramax engine, first introduced in year 2001. Not only that it was a first engine to utilize Bosch common-rail injection, but it also had aluminum multi-valve cylinder heads. Additionally, it managed to be ahead of emissions regulations at that time. And top of all that, it was very reliable and economical. It was no surprise that LB7 was among Ward's "10 Best Engines" in year 2001 and 2002.
LLY replaced the very successful LB7 during 2004, with meeting of emission regulation as the main reason. It featured EGR system and a variable vane geometry (VVG) turbocharger, which provided noticeably favorable performance characteristics if compared to fixed geometry turbo.
LBZ replaced the LLY in year 2006, to who it is almost identical. Only notable change was an increased power output achieved by more aggressive fuel injection tuning. This Duramax variant was used only for a short period time, as it was replaced during year 2007.
LMM was a further iteration of a LLY/LBZ platform, debuted during year 2007 This model introduced a diesel particulate filter (DPF) system, in order to meet Federal emissions regulations. In effect, the LMM was nearly identical to the LBZ, besides the modifications needed to integrate the DPF system. The result was a reduction of particulate emissions by 90 percent. The downside was noticeably higher fuel consumption, much due to the periodic regeneration cycles needed to clean the particulate filter. By this stage, manual transmission was no more available, with Allison 1000 six speed automatic being the only option.
LML Duramax was introduced in 2011, have been engineered using 60% newly designed components in comparison to previous generation Duramax. And as with previous versions, it featured several emission control upgrades. The most important of them was the implementation of SCR system, or diesel exhaust fluid injection. This well proven and simple system helped reducing nitrogen oxide emission levels by 63 percent over LMM powered trucks. Among other improvements was the introduction of the "9th injector" system which supplied fuel for the DPF regeneration directly into exhaust system. All this modifications resulted in not only reduced emissions, but also in increased fuel economy.
Conclusion: Cummins vs Power Stroke vs Duramax - which is better?
You are surely wondering which of these three engines is the best. Let's compare several key features of the current engines:
As you may see, all three engines have roughly similar power output, despite various design and layout differences. What makes them very different is how all this power and torque are delivered and used. With a bit higher power output, Powerstroke and Duramax powered trucks predictably offer a noticeably better acceleration and somewhat better overall pace. Cummins, on the other hand, delivers more torque, especially in the high-output version. With 1.000 lb⋅ft, it is a true leader of the pack when it comes to towing. Also, being a straight-6 engine, it is less complex and in many cases easier to maintain. In other words, Chevy and Ford are faster trucks than an equivalent Dodge, but Dodge outruns them when it comes to towing.
In the end, several words about reliability and running costs. It is frequently argued that modern diesel engines are less reliable and have much higher running cost when compared to older versions. Undoubtedly, added complexity in fuel injection, turbocharging and exhaust treatment systems increases breakdown chances. Still, the main reason for this poor reliability myth lays in that fact that these sophisticated engines are far less forgiving when it comes to negligence. As with all modern engines, the key for longevity lays within proper maintenance. Using high quality fuel, performing regular services using adequate oil and filters and utilizing favorable driving habits will increase the lifespan of any engine.
