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The Best (And Worst) Class 8 Diesel Engines

Written By, Mike McGlothlin

 

Commercial trucks have been roaring up and down America’s roadways for more than a century. During that time, dozens of engines have served duty powering the brands we all know and love such as Freightliner, Mack, Peterbilt, Kenworth, Western Star, and International. Some of these power plants have been good, others great—and some of them haven’t been so great, even ugly…reason enough for us to put together a compilation highlighting the best and worst engines ever produced. Good, bad, and ugly, we at Big Bear Engine—and in conjunction with our sister company, Capital Reman—rebuild many of the Class 8 engines that follow, and we’ve drawn from our 40-plus years of hands-on experience to present this list to you.

 

The Best

 

  1. Cummins 855 Big Cam

Basic Engine Highlights

Earning its name from having a larger-than-average, 2.5-inch diameter camshaft, the 855Big Cam 855 Cummins Diesel Engine Big Cam replaced the 855 ci small cam of the 1960s (which was graced with a 2-inch diameter camshaft). Reliable doesn’t even begin to describe this engine. But on top of being steadfastly durable, it was fuel efficient and powerful. This is the engine that all but singlehandedly allowed Cummins to go from a 29-percent share of the Class 8 market in 1975 to 63-percent by 1983. One of the major high marks for the 855 Big Cam occurred in 1980 with the release of the 475 hp, compound turbocharged Big Cam II, a very big deal at the time.

 

 

Biggest Pros

The list here is long and distinguished, but the 855 Big Cam’s largest selling point in itsClass 8 Semitruck Cummins 855 Big Cam production days was its promise of best-in-class fuel economy. Following the oil crisis of 1973, fuel efficiency became the biggest priority in freight transportation. Next comes the 855 Big Cam’s million-mile reliability, its demand flow cooling system (which only cooled areas of the engine where cooling was needed, thereby saving energy and allowing more horsepower to be used for the task at hand), and its abundance of low-rpm torque. The Cummins 855 Big Cam was also notably more powerful than its small cam predecessor, mainly due to its use of the infamous “Top-Stop Injectors.” Lastly, this engine responds very well to modifications, making it a hit among owner operators.

 

 

Few Downsides

The rap sheet here is short and sweet—just as you’d expect from one of the best dieselCummins 855 Big Cam Reduced Drain Flow Injectors engines ever produced. Cold-start issues are rampant for any Cummins 855 Big Cam operating in cooler climates, but it’s a problem that can easily be solved through the use of a block heater or oil immersion heater. Later models, such as the New Big Cam IV, came with the notorious STC (Step Timing Control) technology and the 88 Big Cam IV—the last version of the 855 Big Cam—had very problematic Reduced Drain Flow (RDF) injectors. Other than that, there are very few negative talking points surrounding this engine platform.

 

 

 

 

  1. Mack E7

Basic Engine Highlights

You can usually judge a great engine by its production run, and the Mack E7—in one formMack E7 Diesel Engine or another—remained on the assembly line for more than 20 years. The fact that its original design, introduced in 1989, survived the emissions crunch of the late 1990s and early 21st century is highly commendable. Even when the E7 morphed into the E-Tech engine later on in its production, they too proved to be million-mile contenders. The E7 is one of the most dependable engine platforms Mack ever turned out, and it proved reliable in a host of applications which, in addition to Class 8 on-highway uses, included construction equipment, firetrucks, agriculture, mining, and power generation.

 

 

Biggest Pros

First and foremost, Mack’s E7 is easy to overhaul. And even when further electronicsMack E7 Class 8 Diesel Truck Engine infiltrated the platform, they were kept relatively simple to diagnose and work on. Part of Mack’s recipe for dependability, solid power output, and clean emissions lies with the proven injection systems the company employed throughout the E7’s lifespan. Mack’s Econovance variable injection timing system, released in the early 1990s, improved fuel economy and horsepower while simultaneously reducing particulate matter (PM) emissions significantly. And let us not forget about Mack’s legendary transmissions, all of which proved virtually indestructible behind the E7.

 

 

 

Few Downsides

The E7 isn’t particularly known for being a powerhouse, but it certainly wasn’t2003 Mack Semitruck ASET Diesel Engine underpowered, either—especially given its size (12.0L). And although it is an extremely reliable, day-in/day-out workhorse, the E7 didn’t always get the best fuel economy. However, it was far from the thirstiest of all the engines of its era. Reliability issues aren’t common with the E7, but the E-Tech models did have their fair share of lifter issues—and an engine brake that many operators felt was a bit on the weak side. Durability did become a problem after exhaust gas recirculation (EGR) entered the mix (on the ASET models) in 2003, which ultimately proved the beginning of the end for the E7 platform.

 

 

 

 

  1. Caterpillar 3406E

Basic Engine Highlights

It was the hot-rod engine of choice for thousands of over-the-road truck owners, and forCaterpillar 3406E Electronic Diesel Engine good reason—CAT’s venerable 3406E was as reliable as it was powerful. It was produced from 1993 to 2007, and most 1990s and early 2000s model Class 8 Peterbilts have this engine in them. Beyond the Class 8 market, many forget that the 3406E (like the 3406A, B, and C versions that came before it) was fairly popular in the marine industry as well, where it was rated as high as 800 hp. Engines with a serial number beginning with 2WS are typically the most desirable, as they were renowned for their stronger block and unbeatable all-around reliability.

 

 

Biggest Pros

By the time the “E” model of the 3406 came along, CAT had all but perfected itsCat 3406B Diesel Engine Class 8 electronically controlled unit injection system. Not only is the 3406E capable of running a million miles before a complete overhaul, but it has the potential to turn out more than 600 hp with a few simple ECM tweaks. Beyond reflashing the ECM, the 3406E responds very well to additional power-adders, and many everyday haulers traversed the country with their yellow engines turning out a four-digit horsepower number. When it is time for an overhaul, the CAT 3406E is a favorite among mechanics, thanks to its ease of serviceability.

 

 

Few Downsides

Weak links are few and far between with the CAT 3406E, but there’s no denying that these3406E Cat Blown Head Gasket engines are notorious oil leakers. Even before old-age sets in, oil seepage is common, especially at the rear of the engine (near the spacer plate and the block). Failed oil coolers are somewhat common (a failure most often discovered by finding oil in the engine coolant), and head gasket issues tend to surface at high mileage (many times from operating the engine while low on coolant). Of course, the price of yellow parts isn’t cheap, either, making most repairs more expensive than what you’ll find on competitor engines.

 

 

 

  1. Detroit Diesel Series 60

Basic Engine Highlights

Class 8 engines don’t get any more legendary than this. Detroit Diesel Corporation’s SeriesDetroit Diesel Series 60 60 epitomizes success in the heavy-duty segment. The O.G. of modern, electronically controlled diesel engines, the Series 60 debuted in 1987—and its Detroit Diesel Electronic Control (DDEC) system proved flawless right out of the gate. John Deere even had a hand in developing the Series 60, with Detroit utilizing its cylinder head design and highly durable piston ring system. Three versions of the Series 60 were produced: the 12.7L (think Class 8), the 11.1L (think motorcoach), and the 14.0L (Class 8) that came out in 2001.

 

 

Biggest Pros

High durability, class-leading fuel efficiency (especially with the 12.7L), and above averageClass 8 Application 12.7L Detroit Diesel Series 60 power epitomize what the Series 60 brought to the trucking segment. These engines are revered for their abundant low-rpm torque and reliable electronics. And despite the 12.7L’s limited displacement, with the proper ECM tweaks it could net just as much horsepower and torque as the competition’s larger engine offerings. Just five years into Series 60 production, it was the most widely used Class 8 engine on the market, having earned tremendous trust among fleet owners and owner operators alike. And in the rare instance when a Series 60 is in need of repairs, replacement parts are usually more affordable than what you’ll find on competing engines.

 

 

 

 

Few Downsides

Even for the formidable Series 60, a few pattern problems exist. At the top of the (short)Leaking Exhaust Manifold Detroit Diesel Series 60 list is the exhaust manifold’s tendency to crack. Loss of power, inadequate turbo boost and drive pressure, and a mess of soot under the hood are telltale signs of this failure. As with any Class 8 engine, spun bearings can become a reality on higher mile engines. On the Series 60, bearing failure is primarily caused by low oil pressure at idle. Prior to 2002, defective wrist pins were found to potentially lead to catastrophic engine failure, but this isn’t a frequent occurrence. On 2003-later Series 60s, EGR-related failures (i.e. cracked EGR coolers and/or EGR valves) are common.

 

 

 

  1. Cummins N14

Basic Engine Highlights

Replacing the coveted 855 Big Cam was not something Cummins took lightly, which is whyElectronically Controlled Cummins N14 Celect Plus the N14 proved to be one of the best Class 8 engines of all time. Often referred to as an “855 with electronics,” the N14 is visibly similar to its predecessor. The modular, 4-valve cylinder heads, unit injection, and even the same oil pan were used. However, the integration of electronics—and specifically, Cummins’ CELECT electronic engine control system—was the big difference, a requirement needed in order to meet modern emissions standards. Production of the N14 began in 1990 for the 1991 model year, and this iconic inline-six was built for on-highway use until 2002.

 

 

 

Biggest Pros

Bluntly stated, these engines are workaholics. They’re both cheap and easy to maintain,N14 Cummins Class 8 Diesel Engine don’t eat you alive on fuel, and are highly reliable, million-milers. Ask anyone that’s spent time around a Cummins N14 and you’ll likely be told it’s the best sounding Class 8 engine to ever storm down the highway. The favorite N14 for many is the CELECT Plus version. Released in 1996, the CELECT Plus produced 525 hp in its most powerful form—and before that respectable horsepower number was made, 1,850 lb-ft of torque was on tap, and available at a low, 1,200 rpm.

 

 

 

Few Downsides

The guts of the Cummins N14 (i.e. its hard parts) are rock-solid, but it’s the injectors thatCummins N14 Diesel Fuel Injector can have their problems. Injector issues and injector harness problems have long plagued early versions of the N14. Its wiring harnesses are known to short out which, in turn, can eventually take out the ECM also. Additional injector-related headaches include leaking O-rings and injector cup failure (where fuel and coolant mix). Oil leaks also tend to develop and seep from the camshaft rocker boxes. In the grand scheme of things, the N14 compares well to the Mack E7: so-so power, great reliability, and above-average fuel usage.

 

 

 

 

 

 

 

 

The Worst

 

  1. Cat 3208

Basic Engine Highlights

It wasn’t exactly a Class 8 engine, but it was used in all kinds of grain and dump trucks, asCaterpillar 3208 V-8 Diesel Engine well as being employed in various vocational, power generation, and even agricultural equipment. Surprisingly, CAT’s 636 ci (10.4L) 3208 could be fairly reliable in a short-haul, medium-duty setting, but then become an absolute nightmare when it was turned loose for long-haul, highway use. In latter applications, a 3208 will almost always find the scrap yard sooner rather than later. Many different versions of this yellow V-8 were produced and, while the naturally aspirated versions are particularly gutless, turbocharged renditions are underpowered, too.

 

 

Biggest Downsides

V configuration engines tend to require higher operating speeds in order to be useful inCat 3208 Diesel Truck Engine vocational duty, and this held true for CAT’s 3208. Unfortunately, in high rpm applications where maximum horsepower is a round-the-clock requirement, such as in heavy tillage agriculture or when powering marine vessels, the 3208 doesn’t last long. In these environments, 2,500 to 3,000 hours seems to be the 3208’s typical service life. On the other end of the spectrum, in reduced rpm applications turning out lower horsepower it tends to last considerably longer.

 

 

 

 

 

Major Failure Points

Given the high rpm a 3208 has to be operated at, combined with the relatively small3208 Caterpillar Parent Bore Block surface area of its rod bearings, rod bearing failure is (not surprisingly) a common occurrence. The problem with this type of failure taking place in an engine with a parent bore (non-sleeve) block design is that it can render the crankcase irreparable. Hence the reason the 3208 earned the nickname “throwaway CAT.” Beyond spun rod bearings, main bearings often meet a similar fate in highly spun 3208’s. Dropped valves also tend to lead to catastrophic 3208 failure, and usually occur in engines that haven’t been properly maintained and that have seen abuse.

 

 

 

 

  1. Detroit 6V92

Basic Engine Highlights

You always know when a two-stroke Detroit is coming, and the Series 92 engines are noDetroit Diesel 6V92 Engine different. Introduced in 1974, Detroit Diesel’s 6V92 utilized a V-6 design, displaced 92 cubic inches per cylinder (552 ci or 9.0L overall), and would come in a variety of forms. In all, 6V92, 6V92T, 6V92TA, and 6V92TTA models were offered, each one utilizing a blower to draw in air for combustion (as is necessary in two-stroke diesel applications). Power ratings ranged from 270 hp (at 2,100 rpm) and 737 lb-ft of torque (at 1,400 rpm) for the blower-only 6V92 to well north of 330 hp and 950 lb-ft for 6V92TTA engines.

 

 

 

Biggest Downsides

First and foremost, you either like the noise or you hate it. Like all old Detroits, the 6V926V92 Detroit Diesel Takeout Engine makes a lot of racket, especially at higher rpm (where it needs to live). Common complaints beyond decibel level include marginal fuel economy at best and the cost of repairs and parts—the latter issues stemming from them being so far removed from production at this point that few mechanics specialize in them anymore. And of course, the biggest gripe for the 6V92: oil leaks. Oil leaks typically stem from the engine’s airbox drains, which need to be opened up at each oil change interval.

 

 

 

 

 

Major Failure Points

Detroit’s 6V92 can produce a few fireworks from time to time, most notably for runningFuel Rack Detroit Diesel 6V92 away. These runaway scenarios are most common on loose (well-worn) engines that are allowed to idle for prolonged periods of time. However, improper running of the fuel rack can also lead to a runaway upon startup. A lifetime of high rpm operation, combined with high miles and operating hours can culminate in bearing failure and a rod leaving the block. Liner seal failure (usually caused from overheating), which is fairly common and that allows coolant to mix with engine oil, is often a contributing factor to 6V92 rod failure. Last but not least, when the 6V92’s gear-driven water pump experiences a shaft bearing/seal failure, coolant infiltrates the crankcase, oftentimes trashing the mains.

 

 

 

 

  1. Paccar MX-13

Basic Engine Highlights

Paccar’s MX-13 is very competitive, power-wise, with larger engines in the Class 8Paccar MX-13 Diesel Engine segment, and it often gets better fuel economy. Its top-rated power output is 510 hp and a formidable 1,850 lb-ft of torque. However, beyond 500,000 miles repairs can become frequent and expensive. For some fleet managers and owner operators, the MX-13 has been a great, reliable alternative to an ISX Cummins or Detroit DD15. Unfortunately, for many others it’s proven to be a nightmare, mainly due to emissions equipment issues, rampant emissions-related sensor failures, electronic headaches, and habitual fuel injector issues.

 

 

 

 

Biggest Downsides

Repairs, and the frequency that repairs are required, are among the biggest drawbacks of Peterbilt 386 Model MX-13 Paccar Diesel the MX-13. The engine’s rear gear train makes camshaft replacement (or any work on surrounding components) more laborious and expensive, diagnosing issues can be extremely time consuming and often requires Paccar-specific software, and replacement parts are on the expensive side. In fact, when weighing the costs of overhauling the original engine vs. complete replacement, many truck owners will opt for a brand-new MX-13. As we alluded to earlier, Paccar’s MX-13 is quickly becoming known as a 500,000-mile engine.

 

 

 

Major Failure Points

Exhaust aftertreatment predicaments are par for the course with all of today’s over-the-Diesel Fuel Injector Paccar MX-13road engines, but the Paccar MX-13 has built a notorious reputation for diesel particulate filter (DPF) and DPF-related issues. Extended idle tends to kill the DPF, while the selective catalytic reduction (SCR) system is plagued by diesel exhaust fluid (DEF) doser valve, sensor, and wiring harness problems. Within the MX-13 engine itself, fuel injector issues are common. And due to the frequency with which they fail, a class action lawsuit was brought against Paccar, which states the injector nozzles were improperly designed and have a tendency to plug up with debris.

 

 

 

 

 

 

 

  1. Navistar MaxxForce 13

Basic Engine Highlights

Meet the engine that nearly bankrupted Navistar. On paper, the MaxxForce 13 had it all:Navistar MaxxForce 13 Diesel Engine sequential turbos, horsepower and torque that rivaled 15-liter engines, great fuel economy, and DEF-free emissions compliance. In reality, most MaxxForce 13’s weren’t on the road long enough to be able to live up to the hype. Warranty claims went through the roof, every truck equipped with one lost most of its resale value, and Navistar’s long-standing reputation was permanently scarred. The company’s hardline against adopting SCR as a NOx-reducing technology (opting instead to meet EPA 2010 standards using EGR alone) proved fatal, and the EGR-only MaxxForce 13 program was quickly discontinued.

 

 

 

 

Biggest Downsides

Downtime and repairs. Repairs and downtime. The average MaxxForce 13 sufferedInternational Class 8 Truck MaxxForce 13 Diesel Engine immense downtime, not to mention the fact that replacement parts were expensive and the engine wasn’t particularly easy to work on. In fact, the compound turbocharged MaxxForce 13 was so tightly packaged that even a simple air compressor job booked for 10 hours labor. Add to that the fact that there were four radiators on the engine (a low-temp unit for interstage cooling, a high-temp unit for aftercooling, a radiator for the first stage of EGR, and a second for the second stage of EGR) and you start to get the idea…

 

 

 

 

Major Failure Points:

EGR issues dominated most MaxxForce 13 service records, the extent of which includesLow Pressure EGR Cooler MaxxForce 13 Diesel ruptured or plugged EGR coolers and failing (or ailing) EGR valves. However, like every other engine in the segment, DPF problems were also routine, with pressure differential and temperature sensors, along with the DPF itself, often requiring attention. To a lesser extent, high-pressure fuel pump failure wasn’t unheard of, a chain reaction type failure that can simultaneously damage the fuel injectors when it occurs. Thankfully, the general structure of Navistar’s MaxxForce 13 was sound, as it was largely carried over in 2013 when the company finally embraced the use of SCR technology.

 

 

 

 

 

 

 

 

  1. Navistar MaxxForce 15

Basic Engine Highlights

The debacle that was the pre-SCR MaxxForce 13 wouldn’t be complete without mentioningMaxxForce 15 Navistar Diesel Engine its bigger, just-as-problematic brother, the MaxxForce 15. Although Navistar billed its EGR-only MaxxForce 15 as the “no-hassle solution” to meeting 2010 NOx emissions standards, in reality EGR was the biggest hassle on this engine. The “Advanced EGR System” billing simply didn’t pan out. Combined with the similar shortcomings of the MaxxForce 11 and 13, Navistar stock dropped like a rock during the 2010-2013 time frame. Its few high marks were that it too made use of high-pressure common-rail injection and that it benefitted from a rather solid structural makeup.

 

 

 

 

 

Biggest Downsides

Based on similar designs, the MaxxForce 15 and MaxxForce 13 shared mirror-like EGR-International Pro Star Class 8 Truck MaxxForce 15 Navistar Diesel Enginerelated failure patterns, and equally excessive repair costs. Also tightly packaged, complete with elaborate cooling components dedicated solely to controlling exhaust gas temperature, the jam-packed, complex nature of the MaxxForce 15 makes it very labor intensive to work on. Adding insult to injury, Navistar’s EGR-only approach did zero favors for the engine’s fuel efficiency. The MaxxForce 15 and 13 are widely and unfavorably known for their poor fuel economy. After just 1,400 were built, Navistar pulled the plug on the MaxxForce 15. A Cummins ISX option would replace it in International trucks.

 

 

Major Failure Points

EGR, EGR, EGR. The very system that was intended to keep Navistar’s leading Class 8Navistar Assembly Plant Navistar MaxxForce 15 engine emissions-compliant ended up being its undoing. In particular, EGR valve shaft failure was common on the MaxxForce 15. It’s a breakdown that essentially allows an unmetered amount of soot to enter the intake, cylinder head, and engine, and that also plugs up EGR coolers and even the exhaust manifold. Additionally, turbo air control valve failure (and intermittent operation) was highly common and led to all kinds of EGR system trouble. Unfortunately, check engine lights (CELs), reduced fuel economy, and a dramatic loss in power became all too familiar and frequent for many MaxxForce 15 owners.

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