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The Best (And Worst) Medium-Duty Diesel Engines

Written By, Mike McGlothlin

 

Vocational vehicles like dump trucks, cement mixers, buses, refuse trucks, and fire engines have been almost exclusively diesel-powered for decades. The engines in these applications are often smaller in size than what you’ll find in Class 8 trucks, but at the same time are notably larger than the power plants used in light-duty vehicles. It’s time for a comprehensive “best” and “worst” list that covers medium-duty diesel engines. From the renowned and nearly-unkillable International DT466 to the dreaded, triple-nickel Cummins V-8, five of the greatest oil-burners ever designed make the cut below—along with a few candidates most would agree belong in the scrap yard.

 

THE BEST

  1. International DT466

Basic Engine Highlights

You can’t have a conversation about the best medium-duty engines of all time withoutInternational DT466 Medium Duty Diesel Enginementioning International’s DT466. The largest of the 400 series of engines developed in the late 1960s, the 7.6L inline-six—which all but introduced wet sleeve technology to the medium-duty segment—proved durable in everything from farm tractors to bulldozers, dump trucks to school buses, and even in military equipment. As a result, the DT466 enjoyed a 45-year production run. Not to be confused with the 466 ci MaxxForce DT that replaced it, the DT466 features a 4.30-inch bore, a torque-friendly 5.35-inch stroke, and was available in both mechanical injection (1971-1993) and electronic injection form (DT466E, 1994-2003).

 

Biggest Pros

Robust and rebuildable help describe the DT466’s solid construction and stellar servicePlateau Honed Cylinder Bores DT466E record. Its plateau-honed cylinders give the bores a perfect finish and its use of wet sleeves means it can be rebuilt out in the field (and with the engine in-frame), not to mention the fact that the sleeves themselves are induction-hardened for wear resistance. Built like an anvil, the 400-pound, deep-skirt, gray-iron block features thick (almost overkill) bulkhead sections, seven large main bearings, and precision line-bored main bearing surfaces. The 180-pound, forged-steel crankshaft boasts induction-hardened journals and fillets, the sizable connecting rods are also made from forged-steel, and six head bolts are employed per cylinder to clamp the head to the block.

 

Few Downsides

DT466E International Diesel EngineThe list here is short and sweet, with the biggest offender on mechanically injected DT466’s being nothing more than the engine’s propensity to develop leaky fuel return lines. When the DT466E model brought HEUI injection into the equation, oil maintenance (or lack thereof) could result in a myriad of issues—but the biggest HEUI-related headaches came about in the engine’s later years, from 2004-2006. Further gripes include the engine’s tendency to overheat when worked hard (and especially when lugged) in hot ambient temps, and the fact that’s it’s heavy and sometimes even accused of being underpowered given its size.

 

 

 

 

  1. Cummins 6CT

Basic Engine Highlights

It might not rival the following the DT466 has, but the Cummins 6CT (also known as the8.3L Cummins 6CT Inline Six Diesel Engine 8.3L Cummins) has definitely earned its place as one of the top medium-duty engines. This 504 cubic inch I-6 diesel possesses a 4.49-inch bore, a 5.32-inch stroke, and a simple, two-valve head design. Like the DT466, it also boasts six head bolts per cylinder for optimum combustion sealing, and mid-stop wet liners, which make in-frame rebuilds very cost-effective. A cast-iron block, forged-steel crankshaft and connecting rods, and mechanical injection make for a stout, reliable, and highly popular engine in various markets. The 6CT was used in everything from farm tractors and combine harvesters to construction equipment and dump trucks, as well as buses, mining, rail, forestry, marine, and industrial equipment.

 

Biggest Pros

Cummins’ 6CT power ratings ranged from 185 to 260 hp and 631 to 889 lb-ft of torque6CT Cummins P-pump Medium Duty Diesel Engine during its production run. By modern standards, these power figures are easily outmatched. However, the 6CTs were solid low-rpm torque producers—a welcomed advantage in dump truck applications where heavy loads had to be lugged up substantial grades—that could also turn out horsepower in reliable, ‘round the clock fashion. From its two-valve overhead design to the fixed geometry Holset HX40 turbocharger to its mechanical injection pump and pop-off style injectors, the Cummins 6CT was a simple, well-built engine. And, with both simplicity and high production volume on its side, parts and repairs remain affordable even by today’s standards.

 

Few Downsides

Case IH 2388 Combine 8.3L CumminsVery few drawbacks are associated with the Cummins 6CT, but chief among them is its tendency to throw rods and window blocks in agriculture applications (and namely combine harvesters). The combination here of high rpm, a restrictive cooling system, low vehicle speed, and a filthy working environment can (predictably) lead to engine problems with even a minimal amount of maintenance neglect. On-highway versions of the 8.3L often live longer lives (thanks to their freer flowing coolant routing), with head gasket failure being one of the biggest concerns as the miles or operating hours rack up. Other common problems come with age and include cracked exhaust manifolds and oil leaks.

 

 

 

 

  1. Caterpillar 3306

Basic Engine Highlights

This yellow inline-six was on Caterpillar’s assembly line from the early 1970s all the wayBig Bear Engines Caterpillar 3306 Remanufactured Diesel up until the late 1990s, if that tells you anything… Cat’s rugged 3306 made a name for itself in dump trucks, cement trucks, and refuse trucks, as well as in various off-road applications like bull dozers (D5, D6, and D7), excavators, and gen-sets. Believe it or not, the 3306 was even used in Class 8 trucks back in the day (although larger, more powerful options usually got the call there), and the 3306B was a huge seller in the marine industry. Cat’s cast-iron 3306 displaced 638 cubic inches (10.5L), featured a two-valve cylinder head, six head bolts per cylinder, and wet sleeves.

 

Biggest Pros

Benefitting from a straightforward, uncomplicated I-6 design, the Cat 3306 is easy to workCat 3306 Surplus Diesel Engineon and service. And while it wasn’t the king of horsepower, it was rock-solid reliable regardless of application and came to work hard every day of the week. Back in its heyday, its biggest competitor was the Cummins L10, another solid engine for its era. By most accounts, the 3306 bests the L10 in every measurable way (i.e. service life, reliability, low-end torque output, and fuel efficiency). Its long, 6.00-inch stroke (bore checks in at 4.75-inch) enables the kind of low-speed grunt that both gets big loads moving and that also conserves fuel.

 

Few Downsides

Caterpillar 3306 Diesel Engine Blown Head Gasket

Everyone that owns and/or operates a Cat 3306 knows that they’re notorious hard-starters—this engine’s biggest gripe, but one that is hardly a cause for concern. On the more serious side, blown head gaskets are a worry with excessive operating hours or miles, but sometimes occur earlier in the engine’s life due to liner protrusion issues. That, and the cylinder head tends to crack when the engine is overheated. Unique to the 3306, it was produced in both indirect injection (IDI) and direct injection (DI) form, the former type being very fuel hungry in comparison to DI versions.

 

 

 

 

  1. Duramax 7800 (Isuzu)

Basic Engine Highlights

The 7.8L Duramax-badged Isuzu is better than you remember it, and here’s why. Inline-sixIsuzu 7.8L Duramax LG4 Diesel Engine design, gray iron block with wet sleeves, high-pressure common-rail injection with Denso solenoid type injectors and high-pressure fuel pump, and up to 860 lb-ft of torque available at just 1,450 rpm. That’s right, Isuzu built a very heavy-duty engine, and one that found a happy home powering Chevrolet Kodiak and GMC TopKicks. Known by RPO code LG4 for 2003-2007 models and LF8 for 2008-2009 engines (as well as the 6HK1, or 6H), the Japanese I-6 could be had in variations ranging from 195 hp up to 300 hp—and 441 lb-ft to the aforementioned 860 lb-ft.

 

Biggest Pros

Ready for the most impressive part about Isuzu’s 7.8L? It boasted a B10 life of 410,0006HK1 Isuzu Medium Duty Diesel Enginemiles. In comparison to the DT466E’s 300,000-mile B10 life (and especially the 220,000-mile B10 life given to GM’s 6.6L Duramax V-8), it was clear Isuzu had done its homework on their medium-duty power plant. Its available 860 lb-ft rating was also big news at the time given that Cat’s 7.2L C7, at 800 lb-ft, was its biggest competitor, and the Isuzu’s highly reliable, Garrett GT3788LVA VGT turbocharger provided great responsiveness. In terms of heft, it was fairly light (at least for a medium-duty diesel), tipping the scales between 1,180 to 1,250 pounds (depending on specific application).

 

Few Downsides

GMC 8500 7.8L Duramax Diesel It sounds like a broken record sometimes, but the 7.8L Isuzu is also one of those engines that was often thought of as being underpowered. Granted, if it had been rated for higher horsepower and torque than 300 hp and 860 lb-ft, it would not have been able to live up to its profound B10 life. For the most part, there isn’t any bad news on this engine. That said, the addition of a diesel particulate filter in the 2008 model year did bring intermittent emissions-related issues into the fold. Of course, EGR cooler leaks were part of the equation, too. Perhaps the 7.8L’s biggest downside is parts availability.

 

 

 

 

 

 

  1. Cummins L9

Basic Engine Highlights

Cummins earns its second spot on our best medium-duty engine list with what is arguably2021 L9 Cummins Diesel Engine Medium Duty Truck the premier power plant in the modern era, the L9. This I-6 platform has more than 25 years of production under its belt (also see the ISL and ISL9)—and 5 million have been produced globally. The Cummins L9 makes use of a 4.49-inch bore and 5.71-inch stroke to displace 543 cubic inches (8.9L). It features a cast-iron block and head (24-valve), utilizes wet sleeves, a forged-steel crankshaft and connecting rods, and six head bolts per cylinder. The L9 also benefits from the use of Cummins’ proven XPI common-rail injection system.

 

Biggest Pros

Thanks to both its size and technology, the Cummins L9 all but owns the bus and motorCummins L9 Top View Inline Six Industrial Diesel Engine coach segment, dwarfing its primary competitor, Detroit Diesel’s 7.7L DD8. Depending on specific application, the L9 can turn out as much as 380 hp and 1,250 lb-ft of torque. And, in conjunction with its XPI fuel system, electronically actuated VGT turbo and single module aftertreatment device (SCR, DEF doser, and DPF, all in one), the Cummins L9 is one of the cleanest-burning medium-duty engines on earth—with highly packageable emissions equipment. But despite its long list of accolades, in 2026 the L9 will be replaced by the X10, at least in North America.

 

Few Downsides

Cummins ISL9 Exhaust Aftertreatment SystemIt comes as no surprise when a late-model engine struggles (sometimes repeatedly) with emission equipment failures. Every diesel engine produced in modern times faces these emissions-related headaches, and the Cummins L9 is no different. While arguably more reliable than most engines on the market, EGR cooler leaks are fairly common, along with the age-old DPF pressure differential sensor issues, which (like virtually every modern diesel engine) tend to surface with regularity. Beyond emissions trouble, sudden engine stalls traced back to faulty ECMs forced Cummins to issue an immediate recall for certain L9 engines back in 2022.

 

 

 

 

 

THE WORST

  1. Cummins VT-555

Basic Engine Highlights

Although Cummins has produced better V-8 diesels in modern times, the VT-555 willCummins V-555 V8 Marine Diesel Engineforever be remembered as either the “Triple Nickel,” “Triple Trouble,” or the “Triple Cripple,” depending on who you talk to. Produced from the late 1960s until 1979, Cummins’ 555 ci V-8 featured a 4.625-inch bore, a 4.250-inch stroke, and produced 235 to 270 hp depending on the application. Both naturally aspirated and turbocharged versions were offered and it was used in buses, heavy transport, marine vessels (where it fared the best), and agriculture. Surprisingly to some, the VT-555 did utilize wet sleeves. However, this bit of heavy-duty technology carried over from its I-6 cousins did little in the way of adding durability to this engine.

 

Biggest Downsides

Perhaps the worst part of the Cummins VT-555 was the conundrum it put the operator in.VT-555 Cummins V8 Diesel Salvage Engine If you ran the engine where it was meant to be operated—between 2,800 and 3,300 rpm—you avoided lugging it at the kind of low-rpm where higher cylinder pressure (i.e. torque) could damage them or, worse, make them scatter. However, operating the engine at high rpm also had the effect of vibrating them to death. In farm tractor applications, those that were worked hard typically only saw 3,500 hours accumulate on the meter. And not only is the VT-555 expensive to rebuild when the time arrives, but parts are scarce due to them being discontinued so long ago. Beyond that, the “triple nickel” is heavy given its size (more than 2,200 pounds in marine trim), it doesn’t tolerate overheating well, and finding coolant in the engine oil is par for the course.

 

Major Failure Points

V8 Engine Block Cummins VT-555 Diesel

Catastrophic engine destruction is common with the VT-555, and a lot of it stems from the use of relatively small main and rod bearings. With roughly half the surface area as compared to inline-six variants, spun main and rod bearings are extremely common—but that’s not all. Broken crankshafts, connecting rod failure (often sparked by a spun bearing), and dropped valve seats are all frequent forms of failure with this 9.1L V-8. A lack of coolant filter and DCA additive maintenance is known to cause the wet sleeves to pit due to cavitation, and any instance of overheating quickly destroys the sleeves’ seals, leading to coolant flooding the crankcase.

 

 

 

 

 

 

 

  1. Detroit Diesel 8.2L

Basic Engine Highlights

If you’re old enough to remember this engine, chances are you remember it well. BuiltDetroit Diesel Fuel Pincher V8 Engine Valve Coverfrom 1979 to 1990, the Detroit Diesel Allison Division 8.2L (also known as the “Fuel Pincher”) was a 500 cubic inch four-stroke V-8 offered in both naturally aspirated and turbocharged form (8.2N and 8.2T, respectively). It served time in on-highway (think buses, Ford F700s and GMC 7000s), power generation, and marine applications—and nearly all of them had problems. The one advantage the 8.2L Detroit had over the competition during the 1980s was its fuel consumption, or lack thereof. Thanks to its “swirl-fire combustion” and unit injectors, even loaded toter homes can squeeze 12 to 15-mpg out of the 8.2L V-8.

 

Biggest Downsides

As with most medium-duty engines, there were several horsepower ratings offered for the8.2L Detroit Diesel Open Block V8 EngineDetroit Diesel 8.2L. However, some left the factory producing a dismal 130 hp. Underpowered would be an understatement and, as a result, most of them spent their entire lives at wide-open throttle. Now for the serious drawbacks. An open deck block design, where the top portions of the cylinders aren’t surrounded by cast-iron (they’re essentially free-standing), led to rampant head gasket issues, and only having four head bolts per cylinder didn’t help matters. Over-hardened cylinders also kept piston rings from seating properly, leading to significant oil consumption. Aluminum fuel transfer pump failures were common. Rocker arm and valve stem pitting was frequent, as was main bearing flaking. And the list goes on.

 

Major Failure Points

Fuel Pincher Detroit Diesel Allison V8

In addition to blown head gaskets, Detroit Diesel’s 8.2L was also susceptible to crankshaft failures thanks to its narrow main bearings (and perhaps the engine’s tendency to add coolant to the engine oil). Cam follower failure was known to be disastrous for them, as was the case when its oil pump bushings spun in the block, requiring crankcase replacement. Rumor has it that 50-percent of all applications powered by the 8.2L had to have their engines replaced at least once under warranty. How the engine survived a decade of production is anyone’s guess, and there were countless updates introduced to try to salvage some reliability out of its design, but eventually GM (parent company of Detroit Diesel at the time) pulled the plug.

 

 

 

 

  1. Cat C7S

Basic Engine Highlights

Caterpillar’s C7 replaced the 3126, which was on its way out in 2003 due to its inability toC7S Caterpillar Common Rail Diesel Engine meet tightening emissions standards set to take effect in 2004. The C7 shares the same, basic, parent bore block and cylinder head as the 3126 (and the 3116 that preceded it, for that matter), but not much else is similar. Along the same lines, the C7S is far from a spitting image of the original C7. In fact, the “S” variant is a very different animal. The common-rail C7S was produced from 2007 to 2009 and drastically broke away from the early, HEUI-injected C7 models in order to comply with 2007 emission standards.

 

Biggest Downsides

Complexity killed the Cat with this one, as the C7S was made overly complicated when itCaterpillar C7S Acert Diesel Engine didn’t need to be. For example, four fuel pressure regulators were fitted to the C7S (vs. a single unit on early C7’s), two separate fuel circuits (with two fuel circuit pressure sensors) and two manifolds were employed for the aftertreatment regeneration device (ARD) head, and the engine’s air compressor was timed with the engine, a very rare arrangement. A complicated, electronically controlled fuel enable valve also made the regeneration process extremely finicky, and a failure-prone CGI system (Cat’s term for EGR) didn’t help matters any. Long story short, “up-time” was often hard to come by with the C7S.

 

Major Failure Points

ARD Head Caterpillar C7 Diesel Engine Emissions EquipmentThe more complicated something is, the more likely it is to experience problems, as was the case with the ARD head on the C7S engine. Problems with the ARD head’s separate fuel circuits and fuel nozzle are rampant, and up on the engine itself injector issues were often hard to accurately diagnose. The problematic clean gas induction (CGI) system often set CEL’s, VGT turbo failure was frequent, and replacement parts weren’t cheap. Add in the fact that most diesel technicians aren’t familiar with this relatively short production run of engines and you quickly realize how expensive it can be to keep a C7S running today.

 

 

 

 

 

 

 

  1. MaxxForce DT

Basic Engine Highlights

This is no DT466, although the uninformed may see the MaxxForce DT’s 466 ciMaxxForce DT Wet Sleeve Diesel Engine displacement and believe it’s a newer version of the old, unkillable 7.6L from the 1970s, 80s, and 90s. It’s not and here’s why. The Navistar-built MaxxForce DT shared the same basic displacement as the DT466, but its bore and stroke are different (4.59-inch and 4.68-inch vs. 4.30-inch and 5.35-inch for the DT466) and—though it too employed wet sleeves and highly robust internal components—it was saddled with a host of NOx and particulate matter curbing equipment that routinely took it off the road.

 

Biggest Downsides

The biggest weak link of the MaxxForce DT exists in its EGR system. When NavistarExhaust Gas Recirculation Cooler MaxxForce DT decided to forego selective catalytic reduction (SCR) and diesel exhaust fluid (DEF) in order to meet 2007 NOx standards, the primary means of complying with federal emissions standards was by using even more EGR. As a result, the MaxxForce DT was plagued by EGR cooler failures, sticking EGR valves, soot-laced and highly contaminated engine oil, and plugged diesel particulate filters (DPF). Adding insult to injury, the engine was laborious to perform repairs on and replacement parts were expensive. In less than a decade, the “DT” name went from being synonymous with long-term durability to highly unreliable.

 

Major Failure Points

2010 Navistar MaxxForce DT Compound Turbo Diesel EnginesTo reiterate, EGR system and DPF issues were extremely common on the MaxxForce DT, but you can also add diesel oxidation catalyst, coolant in oil, and injector problems to the list. Navistar’s use of the G2 electro-hydraulic injection system (a 6.0L Power Stroke style HEUI arrangement) proved highly problematic. The addition of a compound turbo system on 2010 models proved troublesome also, and only further increased the engine’s complexity. Without a doubt, the story of the MaxxForce DT is a sad one. After nearly half a century, Production ceased in 2016.

 

 

 

 

  1. Cummins 6BT

Basic Engine Highlights

Listing the Cummins 6BT as one of the worst medium-duty engines of all time may proveCummins 6BT 5.9L Industrial Diesel Engine to be a head-scratcher for some, but hear us out. While the 5.9L Cummins is a great all-around engine—and is practically legendary in light-duty vehicles such as Dodge Rams—it was notably underpowered in most medium-duty applications it was offered in. School buses, dump trucks and even motorhomes struggled up grades or when heavily laden, and fuel economy was often marginal, at best, due to the strain placed on the engine. At the end of the day, an F800 loaded to its max GVW is going to struggle with a 210 hp 5.9L Cummins under the hood.

 

Biggest Downsides

From the outset, it’s important to note that all of the same drawbacks that exist in theFord F Series Dump Truck 6BT Cummins Diesel EngineCummins 6BT in Dodge Rams are along for the ride in medium-duty trucks. That means you still have the dreaded killer dowel pin (KDP) to worry about, the 8mm diameter, steel alignment dowel that can free itself from the block and destroy the front gear train (and potentially the pistons, head, and block as well). Additionally, the number 6 cylinder that’s notoriously starved for air and coolant flow will eventually become a problem—and sooner rather than later if the engine is constantly worked hard. Finally, the Cummins 6BT’s parent bore block means that rebuilds are much more laborious (and more expensive) than they are with a wet-sleeve engine—the typical candidate called upon in medium-duty applications.

 

Major Failure Points

5.9L Cummins Diesel Engine Coolant Flow

While the killer dowel pin is a worry for any 6BT engine, dropped valve seats are arguably more worrisome, as they’re a lot more common—especially as these engines age. Another common inconvenience (and one that also becomes more frequent with age) is blown freeze plugs. Not the freeze plugs in the block, but rather the internal plugs located within the cylinder head. When these freeze plugs go, excessive coolant loss follows (and is often times misdiagnosed as a cracked block) and mixes with engine oil. Again, the Cummins 6BT isn’t a bad engine, but in a medium-duty work environment it is often outmatched by heavy workloads and can easily be outworked by larger, true medium-duty engines.

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