The word “modern” can be considered to be a swear word among diesel enthusiasts. Today’s diesel engines are known for producing unfathomable power numbers straight from the factory, but they have a reputation for perhaps lower dependability than the diesels of decades past. However, that’s not always the case. Nearly two decades ago, Cummins launched its B6.7 straight-six diesel. This modern diesel engine has proven itself to be so reliable that it’s now the default diesel option for not just Ram pickups, but countless commercial trucks.
If there is a second swear word for diesel enthusiasts out there, it’s probably “emissions.” Modern diesel engines require expensive emissions treatment equipment to meet or surpass strict emissions standards. This emissions equipment, which includes diesel particulate filters (DPF), diesel exhaust fluid (DEF), exhaust gas recirculation (EGR), and other equipment, is great for the health of us all. Everyone deserves to breathe cleaner air!
Sadly, some manufacturers haven’t implemented this equipment well. Any modern Volkswagen TDI owner can tell you tales of emissions equipment failing at the least opportune times, turning the vehicle into a slug that’s trapped in limp mode. I have experience with this. My former 2010 Volkswagen Jetta SportWagen TDI suffered from a DPF failure before 100,000 miles. Then the replacement DPF cracked at around 200,000 miles. What’s worse is that since the DPF wasn’t working anymore, my car’s EGR then got sooted up, too. A dealership quoted $3,000 just to replace the DPF alone, shocking since I paid only $2,950 for the whole car in the first place.
Even vehicles with Cummins engines have had their missteps. One infamous example is the Nissan Titan XD Cummins. This truck had a 5.0-liter V8 diesel that Nissan partnered with Cummins to bring to market. In the real world, this truck proved to be ridiculously expensive to maintain, and there were countless reports of major emissions equipment issues on trucks that had far fewer than 50,000 miles. As I noted in my piece, Car and Driver‘s long-term Titan XD Cummins stranded the magazine’s drivers several times. MotorTrend‘s tester also had notable equipment issues.
Back in November 2024, Lewin also wrote about the serious reliability issues that plagued the 6.4-liter Power Stroke V8 diesel engine that went into Ford Super Duty pickup trucks. As Lewin noted in his report, some people consider the 6.4 Power Stroke to be the “worst diesel ever,” and it’s shocking when you realize that this engine was made only as late as 15 years ago. These aren’t ancient trucks, but rigs you absolutely still see on the road today.
Stories like the ones above have tarnished the image of the modern diesel engine. Some people have found that their diesels gave them years of expensive pain and suffering doled out by an engine that’s supposed to be reliable. Yet, there have been modern diesels that were so bad you were just better off buying the gas engine version of your vehicle and saving your money and your sanity.
One diesel engine that has seemingly triumphed in the modern era is the Cummins B6.7. Launched in 2007, this engine has only grown in popularity as time has marched forward. Today, Cummins says that the 6.7 is its undisputed most popular engine and also the number one diesel engine in the entire medium-duty truck market. This is in part because Cummins has made a modern diesel that could be about as beefy as you can expect in the modern era.
Born From Commercial Trucks
The B6.7 is a part of the iconic Cummins B-Series family. Whenever Cummins talks about why the 6.7 is so reliable, it usually mentions having over four decades of research and experience with the B-Series. That number is important to Cummins because, at least in the diesel pickup truck space, the competition can’t say they’ve been building on the same core platform for as long.
As I wrote earlier this month, the project that birthed the B-Series launched in 1979 after Cummins realized that it had focused too much on diesels for the heavy-duty market. While Cummins was fiercely competitive in the heavy truck market, it was leaving money on the table by not also catering to the medium-duty market.
British engineer and Perkins alum Philip E. Jones led the development of the B-Series, and he reportedly started with a clean sheet. Jones is credited with creating the core of the B-Series; one basic platform that can be used to create straight-four cylinder and straight-six cylinder engines that share common components.
One major factor that Cummins attributes to the continued reliability of the B-Series is that the engine was designed to be a commercial engine from the start. Its application in pickup trucks didn’t come until later. That part is notable because commercial engines are usually designed to take a serious beating for hundreds of thousands of miles while doing all kinds of grueling pulling and hauling.
Years ago, David Tracy interviewed then-Cummins’ marketing director David Goggins, who spoke as to why early B-Series engines like the 5.9 were revered for durability:
He went on to say that the 5.9-liter engine actually originally started as a joint venture between the Indiana-based diesel engine company and Case Corporation, which builds tractors and construction equipment. And indeed, starting in 1984 (well before the 5.9-liter engine ever found itself powering a Dodge Ram), Cummins offered three different variants of the 5.9-liter called the 6B, 6BT (turbocharged) and 6BTA (turbocharged, aftercooled), which served duty in tractors, combines, excavators, road graders, pavement rollers, boats, field sprayers and even school buses.
These are all seriously heavy duty applications. Add that to the fact that these engines were available in dozens of different markets with different climates and work environments (like Russia, China and India), and you can imagine how relatively understressed that Cummins Turbodiesel was once it finally made its way into a Ram pickup in 1989.
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But perhaps even more impressive than its factory torque numbers is the engine’s reputation for longevity. Once you start looking at the mechanical bits, you begin to see just how overbuilt the B-Series engine really is. The block and head are cast iron, the crankshaft and connecting rods are forged, the seven main bearings are massive, and like many heavy-duty diesel trucks, the crank and cam are connected by a steel timing gear—not a chain or belt like you’d find in normal cars and trucks. The Holset turbos are also known to last until the end of time.
It’s not just marketing speak, either. A short Internet search will yield innumerable Cummins B5.9 engines still in use today. You’ll find them knocking on the door of a million miles in school buses, medium-duty trucks, and old Dodge Ram pickups.
More Is More
Cummins says that the 6.7 was born from a need to do more. By the time the Cummins 5.9 bowed out of pickup truck service in December 2006, it had evolved into a common-rail design pumping out 325 HP and 610 lb-ft of torque. But the world had changed dramatically since that engine launched. Diesels faced stringent emissions while buyers demanded even more power. Cummins went back to the drawing board to work on the next evolution of the B-Series.
To create the B6.7, Cummins says it started with the familiar cast iron block architecture and cylinder heads of the B5.9, but upped the displacement with a 4.21-inch bore and 4.88-inch stroke. The idea here was to increase power figures across the board while also maintaining high fuel economy. A rise in cylinder pressure inevitably follows an increase in stroke. To deal with that, Cummins kept with a cast-iron block design with siamese cylinder bores. The block stiffener from the newer 5.9 engines also carried over.
In an older press release, Cummins notes that making the 6.7 wasn’t just turning up the taps:
In January 2007, we introduced 6.7L Turbo Diesel for pickups, bringing a 13% increase in displacement and some exciting new technologies. Out of the gate, it bested 5.9L power output and met stricter emissions regulations.
The new 6.7L Turbo Diesel retains the familiar cast iron block and cylinder heads of the 5.9L, but advances in fuel and air delivery change nearly everything else, especially power ratings and maintenance intervals – now at 15,000 miles. The 6.7L produces up to 900 lb-ft torque and 385 HP. Plus, full torque is reached at a low 1,700 RPM. This makes the ride in today’s Cummins-powered RAM 2500 and 3500 models quieter, smoother and more powerful than ever before. Beyond the benefits of increased displacement, advances in Cummins technology continue to set the bar for power, fuel economy, durability and clean emissions.
Hard NOx Life
At launch, the 6.7 made a claimed 360 HP and 650 lb-ft of torque, a figure that Dodge said blew past the competition. When it came to emissions, Cummins says it introduced exhaust gas recirculation and diesel particulate filter technology, both aimed at lowering NOx emissions and particulate matter.
Here’s a slide from Cummins that illustrates those changes:
As you see above, Cummins claims that in addition to EGR and DPF, the use of a Holset HE351VE variable-geometry turbo and Bosch solenoid-style common-rail injectors firing at 26,000 PSI also helped lower emissions. It’s also said that the HE351VE turbo also enabled the Cummins 6.7’s popular exhaust brake feature, something I covered in my recent review of the latest Cummins Rams.
Other engine changes not readily seen to the naked eye are a transition from forged steel connecting rods to powdered-metal connecting rods.
In 2013, another huge change happened with the introduction of selective catalytic reduction (SCR). This meant that 6.7-equipped pickups from 2013 and on (the feature had already been applied to cab-and-chassis trucks in 2011), truck owners had to fill their rigs up with diesel exhaust fluid (DEF). Thankfully, the addition of DEF did result in the removal of the NOx Adsorber Catalyst (NAC). Likewise, the addition of SCR was touted as increasing fuel economy by 10 percent while increasing torque and also doubling oil change intervals.
The Cummins 6.7 was designed to work with this emissions equipment right from the start. Dodge even had Ram 3500 6.7L Cummins “Bluetec” test trucks on the road back in 2007. Back then, diesel truck magazines praised the Bluetec as being the greenest heavy-duty pickup truck ever. But even the early 6.7s were also considered to be some of the cleanest diesel pickup truck engines ever.
Today, Cummins says that the B6.7 is the largest straight-six diesel that you can buy in the light truck and medium duty market. But not just that, Cummins also brags that the B6.7 is the most popular diesel engine in the medium-duty market. Basically, if you’re buying a medium delivery truck or piece of equipment, you’re probably going to select a B6.7 as your powerplant.
Of course, the 6.7 Cummins also just got another major update, from Cummins and Ram:
Working closely with Cummins, the 6.7-liter High-Output inline-6-cylinder Turbo Diesel engine boasts several improvements in drivability, fuel economy, durability, startability, serviceability, and noise, vibration and harshness (NVH).
A redesigned block and head, robust heavy-duty pistons, new performance intake manifold, a new turbo, larger intake/exhaust valves for improved port flow and a higher-pressure fuel system enhance the diesel’s hallmark durability and efficiency. To make service easier, and improve air flow and performance, the new Cummins engine uses improved intake port geometry for top feed injectors, top load lubrication and integrated fuel filters. Helping customers in extremely cold environments, Ram Heavy Duty trucks equipped with the Cummins High-Output Turbo Diesel use glow plugs.
In short, one huge reason the B-Series remains an iconic engine, even in the emissions era, is that Cummins seemingly did its homework. It started with a hard-working commercial engine and developed it with the times. Cummins reportedly also built its newer engines to work with emissions equipment.
The Good, The Bad, And The Expensive
The 6.7 has a ton of fans, myself included. I’ve had the privilege of driving trucks with older and newer 5.9s and now, thanks to the 2025 Ram HD, I’ve finally gotten to try out more than one 6.7. Assuming a completely stock truck, there’s just a shocking night-and-day difference in everyday performance. Of course, the newer engine makes much more power, but that’s only part of it. The variable geometry turbo really makes the 6.7 feel much more responsive than a standard 5.9, and the exhaust brake is also a wonderful addition that you get in a modern diesel pickup truck.
That being said, I’m partial to the way a 5.9 sounds. For a while, I used to set my morning alarm to the sound of a classic diesel truck cold start. The 6.7 is a powerhouse, but the 5.9 is still music to my ears. Of course, your experience here will vary. Lots of people love how smooth and quiet the 6.7 is, especially the latest iteration of it.
Unfortunately, not everything is perfect. A quick search of Internet forums reveals that the Cummins 6.7 does have some weak spots, just like any other engine.
Some folks with early 6.7s reported having issues with soot buildup in various parts and turbo failures. Many of these issues weren’t easily ignored, either, as they had a knack for putting the truck into a reduced power state. One person has even claimed that their 6.7 truck blew “several” turbos, and in one instance, they had a failure that was so catastrophic that fire spat out of the exhaust pipe. Thankfully, it does seem like many of those early issues petered out as production continued.
Sadly, the issues are not isolated. Reports of plugged or failed DPFs are all over the Internet and seemingly involve trucks of all model years. One person with a 2020 Ram 3500 Longhorn Cummins 6.7 HO with only 26,000 miles reported having major DPF issues every 3,000 miles or so. The EGR and EGR cooler also show up in complaints, as both can get plugged up. DEF errors and issues also crop up on these trucks, too.
Watch YouTuber mechanic Dave correctly diagnose and replace the EGR cooler in a 6.7 Cummins truck:
The emissions issues have been such a huge deal in the Cummins community that many owners “weight reduce” (their words, not mine) their trucks the second they come home from the dealership. Of course, the Autopian does not endorse emissions deletes, and neither does the government. I am just noting that it’s quite a common thing.
Still A Beast
That being said, while there have been countless reports of emissions issues and even some reports of blown head gaskets at high mileage, there have also been promising reports of great longevity. One of the known highest-mileage Cummins 6.7-equipped pickup trucks has over 1.3 million miles, and the truck got most of the way there with emissions intact. Other reports include trucks that have reached over 300,000 miles without any major engine or emissions issues to note.
Honestly, that’s great! One thing to remember is that people don’t often go to Internet forums to brag about how perfect their trucks are, so to see even handfuls of reports of people with reliable trucks with intact emissions systems is great.
With that being said, these failures do happen. When they do, it seems that you’ll likely be paying $3,000 or more to have a DPF replaced. Most of that will be in just the part itself. You can also get EGR valves for under $1,000 and EGR coolers for under $2,000. What that means is that if you’re spending $100,000 on that nice Ram 3500 Longhorn, be sure to have some money set aside for if this stuff breaks out of warranty.
To be fair to Cummins here, that’s just the reality of driving a diesel today. In the past, diesels were advertised as a great way to save a bunch of cash. That’s not exactly the case today, at least, depending on the kind of vehicle you’re looking at. Now, you want to buy the diesel to get the best capability and the most torque to do some really heavy work. Of course, diesels also remain the best choice for so many commercial applications, too.
Well, that’s for now, anyway. The new Cummins B6.7 Octane gasoline engine, which has diesel-like specs but without the finicky diesel emissions equipment, has huge potential. We’ll be watching to see how the medium-duty market and the pickup truck market adapt to it.
Still, the general consensus appears to be that the Cummins 6.7 is a reliable workhorse of an engine, and it will work hard even with stock tunes and stock emissions equipment. That’s part of the reason why the 6.7 remains immensely popular. It’s a great engine right from the factory. Many operators will be happy running them as-is. Sadly, problems will always crop up, but some enthusiasts believe that Cummins engines at least have fewer problems than the competition.
So, we arrive back at the start here. Why is the Cummins 6.7 a legend even in this modern era? To put it simply: The 6.7 is a beast. It’s a great engine when stock, has great tuning potential, and keeps countless school buses, delivery trucks, and pickup trucks doing hard work for years to come. It’ll be interesting to see how people will see these engines in 30 years time. Will they eventually become as famous as the 5.9?
Any good primers on how the emissions equipment works? I’ve always been curious about what, exactly, the DPF and such does.
I’d love to know more about this. I’ve always been under the impression that forged connecting rods were the gold standard for strength. I’m curious what benefits powdered metal brings over them.
F deleted diesels. They’re trading all of our collective health for their own benefit. If an engine can’t be made emissions compliant it doesn’t need to exist.
Here here. If you can’t afford to maintain a diesel don’t buy one. As an owner of a 2011 Express 4500 with a duramax I know from personal experience it can be expensive but I knew that going into the purchase. Today you can save almost $10K by checking the box for the gasoline engine in a medium duty. Don’t plead poverty when it comes time to do the maintenance.
Deleting the emission equipment is no different that taking a dump in the local drinking supply. It is a giant middle finger to your neighbors.
“One person with a 2020 Ram 3500 Longhorn Cummins 6.7 HO with only 26,000 miles reported having major DPF issues every 3,000 miles or so. “
It’s noted that person did 26,000 miles over a period of ~2.5 years. Which is to say they do around 10,000 miles a year. Which is to say they likely do a lot driving that involves shorter trips. Which is to say that’s it’s very likely the truck isn’t driven long enough for a DPF burn-off cycle to complete.
Hence the reason the person is having “major DPF issues”.
Modern diesels can still be good for use cases where the truck is driven hard for longer distances.
But for trucks used occassionally for the odd bit of short-haul driving or for a short daily commute, they are a TERRIBLE option.
A gasoline engine, gas-electric hybrid or a BEV would be a much better option for the type of use case like someone who only puts 10,000 miles/year on their vehicle.
This is so true – and why for commercial vehicles trucks that see a lot of city stop and go are pretty rapidly switching to gasoline. Applications like: school buses, ambulance, box trucks, tow trucks, utility trucks, step vans. Today a diesel only make sense if you are doing a lot of highway miles and especially if you are towing for a lot of highway miles.
Owners are also some of their own worst enemies. You should not idle a modern diesel with emission controls yet I still see people that keeping their diesel idling while they go into a store or pump gas. You also have to use the right oil. Lots of people still use the same old oil they have been using for decades instead of the correct low-ash oil specified by the engine manufacturer. The small particles that form when oil is burned plug up the DPF and unlike carbon cannot be burned out with a regen.
Probably being incredibly pedantic on this but I didn’t think 6.4 Fords ever had the round badge in that picture. Thought that badge came with the first iteration of the 6.7.
But yeah it’s a testament to both the 5.9 and 6.7 Cummins that folks will put up with the rest of the truck to have those motors. These days it definitely seems like all the current gen HD diesels have some issue to deal with though. None of them are truly bulletproof. But they’re all very very good motors.
They did have the round with power stroke written out and 6.4 in the center. My brother-in-law had one.
30 years ago I worked in a ford 6.9/7.3 Diesel Remanufacturing plant. Wow those were simple and good motors. I have lost my interest in Diesel at the point when Diesel became more expensive than gas and the motors were so much more to buy than a gas motor. I am glad they still exist and are getting better.
NVH. One measures noise in decibels (weighted or not) and vibration in terms of amplitudes and frequencies, but what is the measure for harshness?
PS: For someone not interested in trucks and RVs, I always end up reading the articles by Mercedes. Well written and informative.
I for one appreciate that she always does her homework and gives us a great little history lesson with each take.
The 5.9 in the Ram is incredible. It will far outlast the truck it was installed in, at least in more rust prone climates.
That Deboss made me laugh building that Edison Ram Recharger because he said he had never actually driven a 2nd gen but had owned about 15 of them. In rust areas a gen 2 is just a box for a 5.9. They are actually nice trucks especially 96+ but definitely rust issues
Some of us call 2nd Gens “shipping crates” because that’s the cheapest way to get yourself a cummins for your swap.
This article shows that building engines to new regulation standards is tricky. Cummins should be proud of how they did. They should also be embarrassed by that mess that was put in the Titan.
Cummins basically does one thing – engines – so you can see their focus pays off.
The Titanic 5 liter engine inside story would be interesting for sure, they fumbled that one.
It’s not like they didn’t test them, there were Nissan pickups with Cummins Test Truck identifications running all over central Indiana the year before production started. I’d see them every day.
Great comment. Mercedes should do that story and throw it in under the Glorious Garbage banner
I wonder if this is another situation like international and ford on the powerstrokes. The same VT365 and maxxforce 7 seemed to not have as many issues in medium duty applications as their hopped up ford cousins.
Did nissan demand more changes to the 5.0 than cummins had originally designed it for?
Good question.
“The same VT365 and maxxforce 7 seemed to not have as many issues in medium duty applications as their hopped up ford cousins.”
And in bold is the likely reason why the Ford versions had more problems.
Jacking up the power also meant it was likely harder on the emissions and other components.
Cummins has several business units. All focused around engines, but certainly not just engines. Turbos, Fuel Systems, Emissions Systems, Filtration, Engines..
I think the point is that they do not build entire vehicles – so there is a lot of the equation that they just dont touch.
Indubitably. This is true for almost every part in a car. The automaker sources the part from another company that does not build entire vehicles – rather just individual components or assemblies. Engine assemblies are one of the major exceptions to that as many automakers do actually design and build their own.
I worked at Cummins Turbo Technologies in the new product development engineering team while the 5.0 engine and turbo were being developed and validated. There are many outside reasons for the failure, but Cummins also fumbled it.
Then you are just the person we need – what major fumbles do you know about?
Ian Johnson thinks the reason the 5.0 was such a fiasco was because Nissan wasn’t preparing the customers for the type of driving and maintenance that befits a Diesel, and, instead, was selling it as an upgrade for the gasser. So a lot of those trucks were used for short commutes or not maintained properly which killed them.
Meanwhile, Ian has over 250k on his truck and has driven it across the country towing ~6000 lbs multiple times, including yearly pilgrimages from TN to CA and back for the King of the Hammers race. He even plans on doing a major build on it to convert it to a dually in order to continue to transport himself and his 4x4s to events.
I drive one of these with 52kmi. Maintenance is expensive. I wouldn’t call it unreliable by any stretch, but it’s also not the kind of vehicle you can just fuel up and drive the wheels off.
I’m curious what your maintenance expenses are.
I daily one and it’s actually cheaper than the 7.3 it replaced a couple of years ago. 1 less gallon of oil on the 6.7, fuel filters are the same price, and the only odd thing I do is clean the soot out of the O2/nox sensors every oil change. My truck is stock because I live in an emissions area, but it has been cheaper on just about everything and I’m at 270k miles.
This is not to knock you in any way – I am genuinely curious.
So in other words, Cummins anticipated and designed it for emissions compliance from the very beginning, instead of being an afterthought like everyone else. That might help.
I like the 6.7 they run great tons of power but you better replace the grid heater with a better engineered aftermarket one or it will granade. Just like the 6.0 PS needed head studs the 6.7 appears to need the grid heater. The 6.7PS is decent but has pump issues and some other little things that drive people nuts. I think the gen 4 rams with the 6.7 that have had the after market fixes will be a cult classic. It’s just a beefy good truck that rides and drives almost unbelievably well for its size and never issues with power. Gen 2 5.9s are hard to come by and sometimes you cry when you see what has been done to them. Hopefully they don’t get they hands on too many more gen4s with the 6.7s because they are too good to let them be turned into a clown show.