Skoolie Engines Explained; The Best Medium-Duty Diesels

One of the questions most commonly asked about Bus to RV conversions is which engines are the best engines found in school (or other) buses. The short answer is "DT466 or 6CTA", but you probably want more than that, right? Maybe even a lot more. And I'm going to give it to you.

Asking a diesel forum or facebook group which diesel engine is the best is a great way to ignite a holy flame war, and get a whole bunch of information thrown at you in no particular sensible order, but it's not necessarily going to give you any information that you can use. With this in mind, I set out to make a comparison of the diesel engines commonly seen in bus to RV conversions myself, so that I could make some sense of them. Not only is it gratifying to have all of the information lined up in front of you so that you can make intelligent decisions, but it also led me to a lot of other interesting informational sources that will surely be a great help in the future.

My goal was to determine what the "best" engines found in commonly converted diesel buses are, and actually figure out why they are better or worse than other examples, and what can (or should) be done about their problems when present. I wanted to cast a slightly wider net than simply looking at what is in school buses specifically, with information on coaches and shuttles as well. In Latin America I would also be looking at minibuses, but they are sufficiently rare here in the states that I didn't feel they really merited research.

I also didn't bother to track power output, for a few main reasons. The most important is that there's not actually much point to it. All of these engines are available with about the same power output, or can be cajoled into producing it — usually without much work. In general, the light duty engines will be underpowered for full-size buses, but any of the medium duty engines will move them around just fine. The limiting factor often isn't even the engine output, it's the transmission torque input maximum. And finally, it's just too much work to really be worth it. You could literally get enough material for an article not just on each engine family, but each engine model, without even discussing aftermarket upgrade options. I have dozens of spec sheets for various engines with maximums and even dyno curves.

What's still missing from this list? You tell me, but notably, Duramax. I haven't researched them at all. Also, other Detroit engines commonly found in buses, and larger Cummins found in transit buses, and the "triple nickel". And then there's the gasoline engines, although I'm not really very interested in those, to be honest.

Desirable Characteristics

There are some specific common characteristics that nearly everyone can agree are desirable in an engine. Not generalities like "reliable", but specific engine features that are proven to enhance reliability. Inline six cylinder engines are inherently balanced, and run smoother than any other common design. Turbocharged engines are more efficient and make more power than naturally aspirated ("NA") engines. Forged components are less likely to fail, and have less tragic failure modes than cast or sintered (powder metal) examples. Wet sleeved engines are more reliable than dry sleeved ones, and both are easier to rebuild than unsleeved ("parent bore") engines, usually not even having to be removed from the frame. Larger engines tend to be more durable than smaller ones, and larger components likewise.

Equally, fuel and emissions systems are critical points to consider. Mechanical injection pumps are simpler, cheaper, and generally easier to maintain or than electronic systems. Inline injection pumps with one pump per engine cylinder (and thus per fuel injector) are more desirable than pumps with a rotary distributor. Engines without any emissions control systems at all are still common or even prevalent, and much of that equipment is failure-prone and expensive, but diesel buses registered as recreational vehicles are not subject to emissions testing or emissions equipment requirements — except that it remains federally illegal to defeat stock emissions devices. EGR started to become common in 2005, DPFs became mandatory in 2008, and DEF injection has been required since about 2010. All of these emissions systems can fail, and in some cases can strand you, or even destroy the engine (particularly in the case of liquid-cooled EGR.)

Given all of this, the ideal engine from a purely technical standpoint (leaving aside for the moment the questions of parts and service availability, etc.) is one which is:

  • inline, six-cylinder
  • turbocharged
  • mechanically fuel injected, or at least equipped with a reliable fuel system
  • pre-emissions
  • built beefy
Every manufacturer has had hits and misses, and every one has its weaknesses and strengths, but the usual consensus is that the manufacturer whose engines best exemplify these characteristics is Cummins. Let's start there.

Cummins

Founded in 1919 by William Glanton Irwin and Clessie Cummins, Cummins' first major success was with a diesel engine used in railroad switchers in 1993. Cummins was a major engine manufacturer to the US Navy during WWII, and has produced a long line of high-quality and well-respected engines, with occasional mishaps like the "triple nickel" Cummins 555 V8.

All of the best-loved Cummins engines have certain things in common, like forged cranks and rods, aluminum pistons, and six inline cylinders. When turbocharged, they all come with Holset turbochargers. They come with intake heater grids for cold weather starting, rather than glow plugs. They commonly feature heavy parts interchangeability between years. Cummins' support networks are second to none, with the best parts and information availability, and the highest numbers of techs in the field. Cummins works on a real "if it ain't broke" mentality, and these engines have worked basically the same way for decades.

Cummins B Series (5.9 liter)

The Cummins B Series, specifically the 5.9 liter engine, is far and away their best-known and best-loved product, and unquestionably the best-regarded light duty diesel in America. First produced in 1984 for use in CASE agricultural equipment, the 6BT (six cylinder B series, turbocharged) turned out to be an ideal fit for Dodge pickup trucks because of its compact design, where they were used from 1989. Chrysler cleverly chose to ship the engine only with a turbocharger, which meant that it was efficient and powerful, while Ford and GM both elected to originally omit it for cost-cutting purposes and had to retrofit something later. The 6BT was upgraded to have 4 valves per cylinder and electronically controlled fuel injection in the 1998.5 model year, and renamed the ISB. The 5.9 liter ISB was superseded by the 6.7 liter ISB in 2007.

As you'll see when we compare to the competition in this class later on, the 5.9l B engine is built like an absolute brick compared to everything else. It's compact, square, and heavy-walled. Pistons are meaty and serious, and the connecting rods make the ones in the other engine look like absolute toys. Part of that is because they have to do more work, since the other engines are higher-RPM V8s, and they don't have to produce as much power per revolution. But part of it is also that since it's at lower RPMs, the mass doesn't cause as many problems, and they could afford to throw more metal at the situation. Those other engines are also less inherently well-balanced than an inline six, so the B engine really comes out as the clear and undisputed champ of the light duty category for this and other reasons.

In 1994, the Bosch P7100 fuel injection pump ("P-Pump") was adopted for emissions reasons. This became the gold standard for mechanical injection pumps. It features an inline design with one cam-driven pump cylinder per fuel injector, and is dramatically more durable and reliable than the pumps used on the V8 engines, making 1994-1998 6BTs some of the most desirable engines on the planet. When combined with the engine's other advantages, it makes this motor the clear winner.

In general, 5.9 liter B series engines have had only two major flaws. The best-known is that all models through 2002 had a problem called the "Killer dowel pin", in which a dowel pin in the front cover would work its way out over time due to vibration, then drop into the timing gear train and often destroy it. Sometimes lucky owners would find that it had successfully missed the gears, and fallen into the oil pan. The fix is to pull the timing cover (an imposing but simple job) and stake the pin, or install an inexpensive kit to retain it when that is not possible. Some 1991 to 2001 models were built with the "53 block", a weak engine block identified by a number 53 on the casting. Those engines should be avoided altogether.

Cummins C Series

Another collaboration with CASE and released in 1985, the 8.3 liter Cummins C series is to the medium duty diesel engine what the B series is to the light duty, though Cummins has much more credible competition in this category. It is a similar design, albeit at a notably higher displacement, but it is also fundamentally different in that it utilizes wet sleeve cylinder liners. This means not only that heat is more efficiently carried away from the cylinders by the coolant than in the B series, but also that it can literally be rebuilt on the side of the road. This is in my opinion one of the two best engines to have in a full-sized bus, when all factors are taken into account.

Like the B series, the C series started out as the 6C, and was later renamed ISC in 1999 when it became electronically managed. Also like the B series, the C got the P7100 injection pump from 1994 to 1998, making these engines the most desirable despite the 12 valve head. Literally the only notable flaw in the C series overall is a fragile injection pump on the 1999-2003 ISC, known as the Cummins Accumulator Pump System, or CAPS. The pumps can easily be repaired in the field in most cases, however, as the problem is usually limited to the accumulator module easily removed from the top of the pump, and they can be protected with increased filtration.

International-Navistar

Founded in 1902 from a merger of five agricultural equipment firms, International Harvester built a variety of equipment until it eventually divested down to only the truck and engine divisions in 1985, and renamed itself Navistar in 1986. In the early eighties, International-Navistar developed several new engines, many in partnership with Ford — all the Ford pickup truck diesels until recent models have been Navistar engines. These engines were of course also used in other contexts, and appear in full-sized buses.

DT Series

International's biggest win isn't a Ford pickup truck engine at all, it's the DT series, most especially including the 7.6 liter DT466 model. They first used this inline, six cylinder, wet sleeve design in farm equipment like tractors in 1971, and the engine appeared in trucks and buses in 1975. In most particulars it can be compared directly to the 8.3 Cummins, except that it has even better parts interchangeability. Over the years there have been five "generations" of DT466. Of these, the most interesting is the 1993-1997 DT466PLN, which featured two different models of the Bosch P-Pump, P3000 and P7100. These are some of the most sought after engines out there. Production trailed off in later years as the electronic DT466E (1994-2006) was phased in.

Like the Cummins engines, all variants of the DT466 feature forged rods and crankshaft, aluminum pistons, and seven main bearings. They are considered some of the best cold-weather starters, to the extent that Navistar didn't see fit to equip them with any cold weather starting aids but block heaters until the very end of production (when some models got an intake heater.) Despite the lack of the glow system, they usually start readily, and that's just one less thing to go wrong. The DT466 and DT466E really are some of the most desirable engines, and that's where I wanted to start because it's a downhill ride from here, albeit with some thrills along the way. And I'm not even going to get into the Maxxforce DT yet, except to say... don't.

International IDI

The International IDI V8 was spawned from a 1981 partnership between Ford and International to produce a light duty motor suitable for use in pickup trucks. The initial fruit of this labor was the 6.9 liter IDI motor, a fairly reliable motor with generally favorable characteristics like a forged crank and rods, aluminum pistons, and an integrated oil cooler. Unfortunately, the injection pump used is the historical "Roosa Master" from the thirties, in the form of the Stanadyne DB-2. It's a cheap little pump in all senses, and it's somewhat fragile. They also chose not to equip the engine with a turbo, so in order to get decent output one has to turn to ATS, Hypermax, or Banks, all of whom put together kits for this motor.

For 1988, rather than turbocharging, the motor was bored out 2.8mm, to a displacement of 7.3 liters. This was an attempt to outclass at least the GM6.2 liter engine, if not the Cummins, again without the use of turbocharging (and again, all the usual culprits brought out new turbo kits, though the old ones will also bolt onto the new motor without trouble.) And this is where the story goes awry. Because of this extreme boring-out without any other changes being made, the engine had thin cylinder walls that can vibrate up to 0.003" during heavy load. This produced serious cavitation which killed many engines through cylinder pinholing. This was never fixed until the 1993 factory turbo motor, which used an ATS 093 turbo kit with a crushed down pipe (to clear the firewall in the Ford F-Series engine bay) as the factory option. This engine also had strengthened main webs to support the cylinders, and enlarged wrist pins.

All of these engines share the same weak injection pump, which is a common point of failure. The 7.3 IDI non-turbo engine blocks are universally weak, and these motors are genuinely gutless without turbocharging. But these problems aside, the 6.9 liter engine and the factory turbo 7.3 are considered to be simple and reliable workhorses. However, as the 7.3 PowerStrokes have begun to hit the maintenance wall (and require a lot of labor) they have become inexpensive, and that has driven down the value of the Ford IDIs to the point where they are worth approximately nothing. At their best on stock internals, they produce the low end of 7.3 PowerStroke horsepower. They also require special tools to perform injection timing, and less and less diesel shops even know how to work on these any more.

T444e/PowerStroke 7.3

For 1994.5, Ford and Navistar's collaboration continued with the T444e, also known as the Ford PowerStroke 7.3 liter. This was a HEUI-equipped, computer-controlled, direct-injected V8 designed to be produced on the same production lines as the 7.3 IDI. Therefore it uses the same bore, stroke, and cylinder spacing. Aside from the oil cooler (which was upgraded between the 6.9 and 7.3 IDI) every other part is different and improved, at least until the 2000 model year when they cheaped out and went to powdered metal connecting rods. The forged rods are literally the same rods used in the 93-94 7.3 IDI Turbo.

In general this is a fairly reliable design, and the Ford model in particular is notable for being capable of being tuned to deliver absurd levels of reliable power. However, this also tends to require spending absurd quantities of money, involving upgrading basically everything outside of the motor (and frequently, many parts inside of it as well.) And that's a down side to this motor in general; having eight cylinders means you've got two more injectors to replace or rebuild than with the smaller 5.9 Cummins, which can make just about as much power when rubbed together with a big pile of cash.

There are a couple of other problems worth mentioning, though; the HPOP that drives the HEUI system tends to get leaky around 200k, and the UVCH (under valve cover harness) often fries around the same time. Fixing this stuff is not very expensive if you do it yourself, but it is a whole bunch of work, and you will likely need an injector rebuild around the same time. Early Ford 7.3s are also weird about being scanned, and a basic scan tool will often fail to report or misreport many values.

VT365/PowerStroke 6.0

The VT365 is on paper a fantastic engine with its variable geometry turbo, and in the real world a miserable failure of implementation. The cooled EGR commonly leaks down into the exhaust valves and grenades the motor, so the EGR has to be improved or deleted. The head bolts are torque-to-yield which is annoying, and inadequate which causes head gasket failures, so they have to be studded. The early valve train is fragile, and has to be upgraded to the late stuff. The HPOP is flimsy from '03.5-04.5. The FICM that generates the injector voltage overheats. All of this can be fixed... for several thousands of dollars and up. Owners who have taken the trouble and also tuned it up have said it's a good motor, but that's a lot of hassle and a lot of money. If you have one in a build that you're into, you might as well have it bulletproofed, but otherwise this motor should be avoided — so enough said.

GM

General Motors has been building diesel engines almost as long as anyone, and with greater volume than most. They purchased Winton Gas Engine and Manufacturing Company in 1930, and renamed it the Cleveland Diesel Engine Division of GM in 1938 when they founded the Detroit Diesel corporation to build the Series 71 diesel — but enough about that for now. They built a variety of unsuccessful light diesel engines for cars, some of which were based on gasoline engines and some of which were simply designed to be built on the same assembly lines as gasoline engines, before finally getting it more or less right with the 6.2 liter V8 in 1982, which begat the 6.5l in 1992. The 6.2 never appeared in cutaways, and anyway shares most flaws with the 6.5, so we can focus on the latter.

GM 6.5

The GM6.5 is an indirectly injected V8 with an iron crank, steel rods, and aluminum pistons. 1992 and 1993 models use the same Stanadyne DB-2 used on the International IDI, but turning the other direction as this engine uses a chain drive for the cam instead of a gear drive, which incidentally is used by every other overhead valve/cam-in-block diesel engine mentioned anywhere near this article. The GM6.2/6.5 engines are alone in using a chain, and aftermarket gear drives are actually somewhat numerous. The cast iron crank is another characteristic shared with only two other motors mentioned here, one of which is the Navistar VT365 mentioned above. It doesn't seem to be a big problem for that motor at stock power levels, but it's a common point of failure in the 6.5.

This motor is actually plagued with problems, however. The block is weak. The flywheel comes apart. The head cracks. Oil lines fail and starve the engine. Early motors have a weak injection pump, while late motors have an injector driver mounted to the pump, which overheats. All of this can be fixed, but you have to completely rebuild the entire engine to solve the problems — at which point what you've got is the weakest engine in this class. Despite all of this, the GM6.5 and even the 6.2 have a solid following among those who want to swap diesels into gas vehicles, because they share the same bell housing as GM's gasoline engines.

Detroit Diesel

Founded in 1938 for the purpose of commercializing GM's new Series 71 two stroke diesel engine design (which was first installed in buses in 1939), Detroit Diesel has had a long and lumpy road. It was spun off in a joint venture with Penske in 1988, and bought by DaimlerChrysler in 2000. It subsequently built the MBE 900 series of engines for Mercedes-Benz.

Series 71

The real meat and potatoes of the Detroit Diesel fleet, these engines have appeared in practically everything from ships to school buses, and in combinations from six inline cylinders up to 24 in a V. Named for their per-cylinder displacement, these dry sleeve two-stroke diesels require a blower simply to feed intake air into the cylinders — the Roots blower that we iconically see mounted atop engines on hot rods. Detroit considered engines with only the blower to be naturally aspirated, and offered engines with turbocharging as well. The engines are named by the number of cylinders, followed by a V for V-shaped engines, a dash, the series (71) and then any additional letters denoting equipment, like "T" for turbo. Hence a six-cylinder turbo motor is a 6V-71T.

These engines appear in six cylinder form in venerable buses like the Crown Supercoach, or the GM New Look, but also are common (usually with eight cylinders) in coaches from companies like MCI. In general they are very reliable, but they also require fiddling. These engines use an enormously clever unit injector design in which final injection pressure is built within the injector itself, which is driven by a cam. However, this means that each injector must be timed separately. The engines also have numerous opportunities for oil leaks, some of which (like both turbo and blower) can easily feed into the intake and cause a runaway. If you are not comfortable being your own diesel mechanic, you should not own a Series 71.

Series 50/60

Introduced in 1987, the Series 60 is a wet sleeved inline six cylinder diesel of 11.1, 12.7, or 14 liters displacement. The 11.1 liter engine ran only through 1998, but the series continued until 2011. The Series 50 is a four cylinder version of the 12.7 liter engine, with two cylinders lopped off and a balance shaft added. All motors use eight head bolts per cylinder for maximum rigidity, have four valves per cylinder (on an unusual gear-driven overhead cam) and EUI fuel injection with injection pressure provided by cam lobe, but injection timing controlled by solenoid.

These are considered reliable engines, with long lifespans. The four cylinder version has output only comparable to six cylinder engines around eight liters, but the six cylinder versions are plenty gutsy. Low-output motors have cast iron pistons, but all the higher-output versions have a 2-piece piston with a forged steel crown and an aluminum skirt; all motors have forged rods and cranks. EUI injection retains the benefits of mechanical unit injection, without the hassle of timing all of the injectors. But you are unlikely to see them in school buses, with the possible exception of the Series 50.

8.2l "Fuel Pincher"

These engines often appeared in cheap GMC buses, and should be avoided studiously. The block, head bolts, and bottom end are all weak, and they may feature an ether starting system which may have been abused during their lifespan. They were offered with and without turbocharging; non-turbo motors are weak, and turbo motors expose the weakness of the block design — which mimics wet sleeve without actually using sleeves. This means the cylinders stand unsupported, which is a truly terrible idea. There is no up side, and you can't fix them, just don't buy one.

Caterpillar

Ah, Caterpillar. One of the most divisive issues in the engine debate, which always inevitably winds up full of anecdotes about million mile cats. Founded in 1925 from a merger of Holt and C.L. Best, CAT (as they are commonly known) has churned out many engines, a number of which are considered to be excellent workers. However, there is one basic flaw common to all of the CAT engines you will commonly find in buses, which is that they are parent bore. Even the larger engines lack cylinder liners, so the block has to be removed from the frame for machining before it can be sleeved. Another flaw common to CAT engines is that they require a broad variety of special service tools; doing a rebuild can require literally five thousand dollars in special tools that don't carry over to another engine family. CAT is also said to be exceptionally proud of their service literature, and charges premium prices for their documentation.

None of this is to say that Caterpillar engines are bad, there's just nothing about them that justifies the hassle of owning them. They do tend to have slightly more horsepower than other engines of the same displacement, but that is not always the case, and the difference is often small anyhow. Parts and service simply cost more than other brands, and the lack of liners and general cost of service lead many people to refer to them as "disposable motors". A high mileage CAT is a bad bet.

CAT 3116

The 6.6 liter 3116 is where much of CAT's bad reputation is founded. These engines are expensive to work on, have fragile injectors, and tend to burn exhaust valve #5. They also had a rash of bad blocks made in France (the "FASP" blocks) which tend to fail under load. Just avoid this motor.

CAT 3126

Based on the 3116, but with most of the big problems solved, and up to 7.2 liters. Still parent bore, though, so wouldn't you rather have a DT or an 8.3? Anyway, the 3126 went through a bunch of revisions, the interesting ones being the "A" (really just the 3126), B, E, and C7. The C7 is considered to be a bit of a lemon, with a bunch of problems in the electronics, but the other motors are basically solid. You could do worse than a 3126, but you shouldn't pay Cummins or DT money for one.

CAT 3208

The 3208 is a V8 design that dates all the way back to 1975, which they kept going until 1992. A 10 liter V8, they appear in some of the larger buses. They too are parent bore, which is fairly unusual for engines of this size, and over the years was offered with and without turbocharging. Naturally aspirated models had a cast iron crank, but the turbo motors got forged steel. This is generally considered to be a credible motor, while it lasts.

Mercedes-Benz

Mercedes needs no introduction, having invented the motorcar. The MBE 900 products commonly seen in surplus buses are actually Detroit Diesel products, and there have been a couple of revisions of them. In general, anything you might say about Caterpillar, you could equally say about Mercedes. They require expensive parts, tools, and documentation, and they are parent bore engines. They are essentially designed to be run once, and then discarded.

On the other hand, genuinely Mercedes-built small diesels are generally excellent, if not without flaws. The 5 cylinder 2.7l diesel used in the early Sprinter is somewhat prone to injector seal leak, and some assorted turbo boost leaks, as well as harmonic balancer failure. However, none of this is difficult to remedy if caught before it progresses too far. And the older (Model 309) buses occasionally seen here can feature the OM617 5-cylinder diesel, which is also excellent — and can easily be swapped for the legendary OM617.951 from the 300SD.

MBE 906

The Mercedes MBE 900's first generation, the 906, is somewhat plagued with problems. This 6.4l straight six offers a plethora of features like an (optional) integrated exhaust brake, but has been known to have problems with injector lines cracking, the air compressor disengaging from the gear, EGR temp sensor failure, weak head bolts, and cracking plastic fuel lines causing fuel starvation. The engine internals seem strong, but a lack of attention to details resulted in a fragile system.

MBE 926

The 7.2l (in six cylinder form) successor to the 906 allegedly addresses most of the problems with the original, except being expensive to maintain.

Conclusion

Some of the motors I've left out, like the Duramax, are important. Some of these motors I have little to say about, because I'd rather not make things up, and I don't have much to say about them. I will have to come back and improve this page later.

Footnote

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