Written By, Mike McGlothlin
We’ve published best and worst lists for both heavy-duty (Class 8) and medium-duty (Class 6-7) diesel engines, but what about the power plants that can be had in domestic pickup trucks? The engines that help get us to work, tow our toys, plow our parking lots, or put in long hours at the farm. Now it’s time to scrutinize the best diesel engines for trucks and the worst light-duty diesels produced over the past 40 years. At the top, you’ll find several first-rate candidates from Cummins, GM, and Ford. Then at the other end of the spectrum, we’ll present some of the most troublesome and disappointing oil-burners to ever leave the assembly line.
The Best
Cummins 6BT
Basic Engine Highlights
Throughout the 1990s, the Cummins 6BT dominated within the diesel pickup market. 
Combined with the new look of the Dodge Ram for ’94, Chrysler topped GM in truck sales and also nipped at the heels of Ford’s perpetual lead—something the automaker’s Cummins diesel engine option played a huge part in it. Introduced in 1989, the 6BT brought direct injection and turbocharging to the pickup truck segment, and from ’94-’98 the Cummins was graced with the coveted Bosch P7100 (P-pump) injection pump. The 359 cubic inch inline-six also brought 400 lb-ft of torque to the table, an output that dwarfed the competition’s torque output at the time.
Biggest Pros
Overbuilt in virtually every way, the 5.9L Cummins boasted forged-steel I-beam 
connecting rods, a forged-steel and heat-treated crankshaft (with 14mm diameter main cap bolts on ’89-’97 model year engines), and six head bolts per cylinder. Additionally, and just like a heavy-duty Class 8 engine, every accessory is gear-driven—and driven via six austempered, ductile-iron helical gears that all underwent heat-treatment for improved strength. Finally, the Cummins 6BT was equipped with a simple, reliable mechanical injection system. In fact, versions of the 5.9L Cummins that are fitted with the Bosch P7100 can be made to double their factory horsepower and torque output with ease.
Few Downsides
Even though the Cummins 6BT is tough as nails, it does have a few shortcomings, the most alarming of which is the killer dowel pin (KDP). At the factory, a steel dowel pin 8mm in diameter was installed in the block to properly align the front gear cover. This dowel pin, which is present on all ’89-’98 5.9L engines, can back out of the block and drop into the cam gear, often spelling disaster. Other drawbacks are nowhere near as severe or crippling as the KDP, but include overflow valve failure on P-pumped versions (again, ’94-‘98), dropped valve seats on high-mile engines, and the fact that the 5.9L is somewhat underpowered when used in medium-duty applications.
6.6L Duramax V-8
Basic Engine Highlights
Like the Cummins 6BT, General Motors’ 6.6L Duramax—a joint effort between GM and 
Isuzu—brought several firsts to the diesel pickup segment. The 90-degree V-8 featured a bore of 4.06-inches, a 3.90-inch stroke, a 17.5:1 compression ratio, and debuted high-pressure common-rail injection as well as aluminum cylinder heads. Released for ’01 model year Chevrolet Silverado and GMC Sierra HD’s, it also represented the first diesel engine in its class to turn out 300 hp. GM’s LB7 RPO code Duramax was quiet, clean, and mean, packing a segment-leading 520 lb-ft of torque to go along with its 300 ponies.
Biggest Pros
Despite introducing high-pressure common-rail injection (an avant-guarde technology in
the summer of 2000) and facing fuel injector issues on the LB7, in time the Duramax proved highly reliable. Highlights of its general engine makeup include a deep-skirt, cast-iron block, induction-hardened cylinders, the aforementioned aluminum heads (which featured four valves per cylinder and that anchored to the block by way of six head bolts per cylinder), and a forged-steel crankshaft that’d been treated to extensive fatigue strengthening. Without upgrading any internal hard parts, the LB7 Duramax can support double its factory horsepower and torque rating, and late-model GM HD trucks can be transformed into 500-rwhp hot-rods with ECM tuning alone.
Few Downsides
Although fuel injector problems were a real concern with the LB7 Duramax (LB7)—an issue that GM offered an extended warranty for—injection system headaches were all but eliminated on subsequent versions (namely the LLY, LBZ, and LMM’s). Speaking of later renditions of the Duramax, the LLY (produced from mid-2004 to mid-2005) suffered from warmer coolant temps and head gasket issues. After that, piston failures sometimes surfaced on LBZ and LMM engines (’06-‘10), usually due to added horsepower, and emissions equipment-related failures became commonplace following the release of the EGR, DPF, and SCR-equipped LML. All of that said, the Duramax platform as a whole has epitomized just how dependable, powerful, and clean a modern era diesel engine can be.
Common-Rail 5.9L And 6.7L Cummins
Basic Engine Highlights
Because there isn’t room for six “best of” diesel engines on our list, we’re combining two
juggernauts here: the 5.9L and 6.7L Cummins I-6’s found in Ram trucks from 2003 to 2018. Full disclosure: the ’19-newer 6.7L is left off due to its hydraulic lifter failures but is an otherwise robust engine as well. The 5.9L used from ’03-’07 possessed the same bore and stroke of its ’89-’02 predecessors but debuted high-pressure common-rail injection. The 5.9L common-rail also utilized a stronger crankcase than the ’98.5-’02 version and turned out 325 hp and 610 lb-ft of torque before it was retired. The 6.7L Cummins boasted a larger, 4.21-inch bore and longer, 4.88-inch stroke, and offered significantly more horsepower and torque, especially as the years wore on.
Biggest Pros
Lightyears ahead of the VP44-fueled 5.9L ISB engine, the common-rail 5.9L was quieter,
cleaner, more powerful, and significantly more drivable. The last of the “emissions-free” Cummins power plants in Dodge trucks, it would also prove rock-solid reliable. Technology took a huge turn when the 6.7L was released, the engine boasting a variable geometry turbo (VGT) that provided exceptional low-rpm response, an exhaust brake (i.e. turbo brake) that vastly improved the towing experience, and 650 lb-ft of torque right out of the gate. Thanks to its larger displacement, the 6.7L has the ability to spool a much larger fixed geometry turbo than the 5.9L, but both engines have tremendous performance potential and enjoy plenty of aftermarket support.
Few Downsides
There is no infamous engine devastation to report with either the common-rail 5.9L or 6.7L Cummins. However, due to the higher cylinder pressures (i.e. torque) it sees, the 6.7L does suffer blown head gasket issues more frequently. On either engine, the electronic (solenoid-actuated) common-rail fuel injectors don’t last as long as the older, mechanical versions did, with the 5.9L’s injectors typically going 180,000 to 200,000 miles before replacement is required. The bulk of the 6.7L’s problems stem from its emissions-curbing equipment. From the EGR valve and cooler, to the DPF, to the NOx absorption catalyst (NAC), to the SCR system, both component and sensor failure is rampant.
6.7L Power Stroke
Basic Engine Highlights
The 6.7L Power Stroke was developed in-house at Ford Motor Company, and it’s a great
engine because it had to be. As you’ll learn later, the two Power Stroke V-8‘s that preceded the 6.7L were plagued with issues both big and small, so Ford did its homework with this engine, which has proven highly reliable over the past 14 years. The 6.7L Power Stroke has a 3.90-inch bore, a 4.25-inch stroke, and displaces 406 cubic inches. It boasts a deep-skirt, compacted graphite iron (CGI) block, reverse-flow cast-aluminum heads, and an air-to-water intercooler—all of which were segment firsts. Its same, basic overall architecture that—released in the summer of 2010 for ’11 model year Super Duty’s—is still in use today.
Biggest Pros
Just like the Cummins and Duramax offerings in the pickup truck segment, the 6.7L Power
Stroke uses high-pressure common-rail injection, so it’s no surprise that it too benefits from being quieter, cleaner, and more powerful than the diesels of old. The engine’s VGT, mounted at the rear of the lifter valley, is driven via exhaust manifolds located in the lifter valley thanks to the reverse-flow cylinder head design. This leads to quicker turbo response and better overall engine efficiency. Always a torquey engine, the 6.7L Power Stroke debuted with 800 lb-ft (and 400 hp) in 2011. In its most recent form, the high output version released for 2023, the 6.7L Power Stroke belts out an incredible 1,200 lb-ft of torque, and 500 hp.
Few Downsides
As is sometimes the case with a fresh engine platform, some mechanical quirks surfaced with the early 6.7L Power Strokes. Among them was turbo failure on ’11-’14 models, whereby the factory, dual compressor VGT was prone to overspeeding and taking out the ball bearing center section. Bothersome valve issues came to light on select ’11 model year engines, but Ford quickly rectified the situation by 2012. However, one problem that surfaces from time to time on all 6.7L Power Strokes is high-pressure fuel pump failure. When this happens, the injection pump (a Bosch CP4.2) self-destructs and can take the injectors out with it. Luckily, roughly only a 1-percent failure rate is associated with this particular failure.
Cummins 4BT
Basic Engine Highlights
Sixty-six percent of a Cummins 6BT has to be a good thing, but that’s only half the story
with the 4BT. You see, it fits where the 6BT can’t, and that makes it the quintessential diesel swap candidate for Jeeps, compact pickups, SUVs, and even cars. The direct injection, typically turbocharged, 239 ci (3.9L) sports the same 4.02-inch bore and 4.72-inch stroke the 6BT does, but packages much better in applications where space is limited between the grille and firewall. While dozens of different CPL (control parts list) numbers exist for them, most 4BT’s came with the 105 hp, 265 lb-ft of torque rating.
Biggest Pros
Highly beneficial to anyone looking for a tough-as-nails engine, the Cummins 4BT uses
many of the same, robust parts employed on its bigger brother. Among them, the connecting rods, pistons, injectors, head bolts (six per cylinder), and main bolts are all the same. The original, 8-valve 4BT’s are almost unbelievably durable, and thanks to its mechanical injection system it’s a true doomsday engine. The 4BT boasts great fuel economy potential, engines are relatively plentiful, it enjoys high parts availability, and it can remain reliable even at power levels well beyond stock. Finally, there are no glow plugs (or glow plug controller) to fail, as the simple 4BT only uses an intake air grid heater to aid startups and limit emissions when cold.
Few Downsides
Without question, the biggest gripe about the Cummins 4BT boils down to NVH—noise, vibration, and harshness. These engines—which, due to their inline-four design, don’t enjoy being inherently balanced like the I-6 6BT does—are rugged, primitive, and loud. Of course, “loud” is relative, but if you’re replacing a gas engine in your swap project with a 4BT, you may be disappointed. Like the 6BT, the 4BT also carries the fear of the KDP (killer dowel pin) working itself loose and wreaking havoc within the front geartrain. Cracks develop in the cylinder head with age and dropped valve seats can occur when age meets excessive EGT. Lastly, the 4BT can be a bit underpowered when swapped into a full-size truck—especially one that’s expected to tow heavy.
The Worst
Basic Engine Highlights
Designed to replace the venerable 7.3L Power Stroke, the Navistar-built 6.0L Power Stroke
had some big shoes to fill. The smaller displacement V8 was nowhere near as reliable as its predecessor, but it was notably more powerful. It still made use of a hydraulically actuated injection system (this version of the HEUI system supplied by Siemens rather than CAT), but higher peak injection pressure led to increased horsepower (325 hp) and torque (560 lb-ft, then 570 lb-ft later on) and cleaner tailpipe emissions. The 6.0L Power Stroke—which featured a 3.74-inch bore, a 4.13-inch stroke, and displaced 365 cubic inches—also possessed a higher compression ratio than the 7.3L at 18.0:1, as well as four valve per cylinder, cast-iron heads. Navistar built more than two million of them.
Biggest Downsides
With an inordinate amount of 6.0L Power Stroke engines, owners often seem to be 
perpetually waiting for the next downtime-induced failure to occur. If it isn’t one thing, it’s another with the 6.0L, but it all starts with head gaskets and EGR. A lack of head fasteners and head bolt size leads to blown head gaskets, while the EGR system (one of Navistar’s first mass produced engines with this technology) was plagued with issues, from minor quirks to catastrophic failures. To a lesser degree, the revvy nature of the 6.0L Power Stroke—an engine that loves higher rpm—isn’t an impressive performer when lugged.
Common Failure Points
The 6.0L’s list of factory shortcomings is long, so we’ll dive right in. Blown head gaskets, thanks to just four torque-to-yield head bolts per cylinder (and each only being 14mm in diameter), are highly common. However, even more common than that are EGR and oil cooler failures. On the EGR side, the EGR valve is prone to gumming up, sticking, and hindering engine performance. As for the oil cooler, it’s miniscule coolant passages are highly susceptible to plugging up, which leads to super-heated engine oil and lack of coolant flow to the EGR cooler (often causing it to rupture, spawning the 6.0L’s infamous EGR cooler failure). Further failure points include corrosion-related VGT issues, injector stiction, the high-pressure oil pump checking out, high-pressure oil system leaks, and a dead fuel injection control module (FICM).
The “Olds 5.7L” (LF9)
Basic Engine Highlights
GM’s 5.7L diesel V-8 is perhaps best remembered for use in Oldsmobile Cutlass and Delta
sedans, but it was also offered in Chevrolet C10’s from ’78-’81 and GMC C1500 pickups during the same model years. Its full production run spanned from 1978 to 1985, but earlier versions (specifically the ’78-’81 mills) are the most problematic. The “Olds 5.7L” was designated with the LF9 RPO code, utilized a 4.06-inch x 3.38-inch bore and stroke, and was loosely based on the same, general architecture of the automaker’s 350 ci small-block gas V-8. However, the LF9’s cast-iron “DX series” block released in 1982 was notably thicker than the gasoline version.
Biggest Downsides
Thoroughly underpowered, the indirect injection and naturally aspirated Olds 5.7L
produced just 120 hp (at 3,600 rpm) and 220 lb-ft of torque (at 1,900 rpm) when it debuted. To make matters worse, from ’82-’85, power figures shrunk to 105 hp and 205 lb-ft—supposedly in an effort to improve reliability. Acceleration throughout the rpm range was sluggish, noisy, and uneventful. Due to budget constraints, the 5.7L wasn’t equipped with a traditional water separator from the factory—a cost-cutting measure that would come back to bite GM, namely in the form of corroded, rusting injection pumps failing left and right. The engine’s one upside was its conservative fuel consumption.
Common Failure Points
In an effort to spend as little money as possible on the 5.7L diesel program, GM chose not to retool and instead left the gas 5.7L head bolt design (size and pattern) unchanged, despite the diesel being exposed to higher compression and cylinder pressures. This predictably resulted in head gasket issues due to head bolt failures. Injection pump failures, attributed to the aforementioned lack of a water separator, were highly common, and came at a time when the quality of U.S. diesel fuel was poor and often water-laced. On the catastrophic side, cracked crankshafts took many engines off the road. On the milder side, glow plugs that ceased to work were extremely common.
6.4L Power Stroke
Basic Engine Highlights
On the heels of the warranty fiasco that was the 6.0L Power Stroke, Ford turned to
Navistar’s 6.4L platform to help revive the Power Stroke name. In some cases, the 6.4L delivered. In other areas, it fell short. The first Power Stroke V8 to utilize high-pressure common-rail injection, the 6.4L featured a bore and stroke of 3.87-inches x 4.13-inches, and came with a sequential turbo arrangement (i.e. compounds) straight from the factory. Its cast-iron block and heads were machined to use larger, 16mm diameter head bolts, which eliminated the blown head gasket scenarios the 6.0L was notorious for. Early on, the 6.4L Power Stroke received generally good reviews, but that would change in time.
Biggest Downsides
Once you’ve breached 100,000 miles—and certainly once you’ve reached the 150,000-mile
mark—all bets are off with the 6.4L Power Stroke. Failures, from minor annoyances to major issues, can surface at any point. Adding insult to injury, the 6.4L is highly laborious to work on, often requiring the cab of the truck it’s powering to be pulled to access parts—and those parts can be (and usually are) very expensive. In short order, what initially appears like a simple need for a replacement part of component can cost a small fortune. A high percentage of these engines experience a cataclysmic level failure before 200,000 miles.
Common Failure Points
The most worrisome failures associated with the 6.4L Power Stroke are cracked pistons and self-destructing high-pressure fuel pumps. The latter component, the Siemens-built K16 VDO pump, can also take the engine’s expensive piezoelectric injectors with it when it goes—and the K16 is difficult to access being that it’s driven off the rear of the engine. Other high-frequency problems, though not catastrophic but costly to address, come in the form of leaking turbo up-pipes and radiators. On the emissions front, the regeneration cycle associated with cleaning the diesel particulate filter is known for diluting the engine oil—not to mention that DPF failure is common. EGR valve and cooler issues are also everyday issues faced by 6.4L owners.
6.5L Detroit Diesel
Basic Engine Highlights
It was supposed to be a bigger, better, more powerful version of the 6.2L it replaced. But
while GM/Detroit Diesel’s 6.5L V-8 did turn out higher horsepower and torque, it ultimately proved significantly less reliable. It was first introduced in natural aspiration form in 1992, but most 6.5L Detroits—especially those assigned RPO codes L56 and L65—were turbocharged. The 90-degree, indirect injection V-8 featured a 4.055-inch bore, a 3.82-inch stroke, and displaced 395 cubic inches. Compression ratio checks in between 18.0:1 and 21.5:1 depending on engine model year and application, and power figures range from 180 hp to 215 hp, and 360 lb-ft to 440 lb-ft.
Biggest Downsides
Cursed with hard part, electronics, and injection pump issues, the 6.5L Detroit failed to
match the relative reliability the 6.2L enjoyed. And even though it turned out higher power ratings than its predecessor, the 6.5L is notably underpowered as compared to the competition. Despite most models benefitting from a turbocharger, its remote-mount style (i.e. not in the traditional location in the valley) facilitated lag issues due to heat loss. And speaking of heat, most 6.5L’s are prone to overheating, due in large part to an undersized water pump and engine fan being used on early versions.
Common Failure Points
A split block and a cracked cylinder head is never an easy fix, nor is a broken crankshaft—all of which are fairly common afflictions for the 6.5L. On ’94-’00 model year power plants, the electronic Stanadyne DS4 injection pump is prone to cavitation, and the pump mounted driver (PMD) on top of the DS4 is apt to failing, usually due to vibration and heat. Further fueling issues unfold when an ailing oil pressure switch, which is in charge of telling the lift pump when to turn on and off, starves the DS4 pump of fuel. Other common setbacks include failed glow plugs, glow plug controllers, and even ECM’s.
2.2L Perkins
Basic Engine Highlights
If you blinked in the 1980s, you would’ve missed the 2.2L Perkins’ short tenure in
domestic compact pickup trucks. The four cylinder engine, model 4.135, was a joint venture between Perkins and Mazda, first debuting in Mazda’s B2200 for the ’82 model year. In ’83 and ’84, it was an option in Ford’s brand-new Ranger. Displacing 135 cubic inches, the 2.2L Perkins I-4 featured a 3.50-inch x 3.50-inch bore and stroke, a 22.0:1 compression ratio, indirect injection, and was only available in naturally aspirated form. Despite its small displacement, the engine was gear-driven rather than equipped with a timing belt (or chain).
Biggest Downsides
In a word: gutless. Granted, it was the early 1980s, when the Corvette packed an
underwhelming 205 hp, but with just 59 hp and only 90 lb-ft of torque on tap the 2.2L Perkins was strictly a fuel economy option. And sip fuel they did, enabling a Ford Ranger equipped with one to return 41-mpg highway and 33-mpg in combined city/highway driving. Due to its abysmal power rating, top speed for a Ranger checked in between 55-65 mph, depending on transmission and gearing. Finding parts for the short production run Perkins is usually one of the biggest obstacles in keeping one of these on the road.
Common Failure Points
The 2.2L Perkins isn’t known for blowing apart, but cracking the cast-iron head is possible if coolant temps are allowed to get out of hand. The head gasket can quickly become an issue with the addition of a turbocharger. These engines came naturally aspirated and with very high compression, so adding forced induction is ill-advised. The little Perkins’ more minor setbacks include glow plug and glow plug controller failure (also a common occurrence for the IH-built 6.9L IDI V8 diesel found in the F-250 and F-350 of the same era) and lift pump failure. As stated, its biggest downfall was its lack of power. After the 1984 model year, Ford replaced the 2.2L Perkins option with a turbocharged 2.3L diesel from Mitsubishi.
