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5.9 Cummins Engine Profile

By September 15th, 2021No Comments

The 5.9 Cummins engine is a common engine found in skoolies and selected as the engine of choice by many soon-to-be skoolie owners. This is one of the desirable engines due to the high power from a small engine for mid-sized skoolies, availability of parts, the life expectancy of the 5.9 Cummins, and several other factors.

In this guide, we will cover a complete overview of the 5.9 Cummins specs, history, problems, life expectancy, maintenance, and comparisons to the popular DT466 / DT446 / Maxforce DT and the T444E.

5.9 Cummins Engine Overview

5.9L Cummins Engine

Cummins first began producing its B Series diesel engines in 1984 for use in agricultural equipment. By the end of production in 2007, the Cummins 5.9L was a popular engine for Types C and D school buses, which carry up to 80 students. Other versions of the diesel engine were featured in Dodge pickup trucks; thus, the B Series diesel engines have strong brand associations with RAM medium- and heavy-duty trucks.

Cummins began producing the engines, including the 5.9L, in Rocky Mount, North Carolina. Later production was expanded to other assembly plants in the U.S. (Indiana), Mexico, Turkey, and the United Kingdom.

5.9 Cummins Engine Models

The first-generation diesel engine model, known as the 6BT or Cummins 12-valve 5.9L, became a popular alternative to the large gasoline V8 engines normally found in full-size pickup trucks. Despite its smaller size, the turbocharged Cummins diesel engine could produce significant power with decent torque at low engine speeds.

5.9 Cummins Turbo Diesel

Cummins diesel engine series went through several design changes as the manufacturer complied with a sharp rise of anti-pollution laws, particularly in the U.S. Federal, state and local governments rolled out environmental regulations aimed at improving air quality for vehicle occupants and communities. Cummins worked with other manufacturers, such as Blue Bird and Thomas Built, to build school buses that would be safe and clean enough to transport students.

In 1998, Cummins replaced the 6BT with the 24-valve 5.9L ISB (Interact System B), which was designed for improved fuel economy and emissions. The ISB model was one of the largest straight sixth engines used for light truck vehicles and school buses, and the improved high output 600 version was on the Ward’s 10 best engines list for 2004.

The ISB model used electronically controlled Bosch fuel systems, unlike the 6BT systems which were mechanical. The 1996-1998 California model engines were fitted with exhaust gas recirculation (EGR) to meet the California Air Resource Board’s (CARB) stringent Nitrogen Oxides (NOx) standards. For further environmental protection, bus operators shifted to biodiesel fuels. Some high horsepower Cummins engines with certain fuel systems can use industry-approved biodiesel but check with a mechanic or Cummins representative before determining if your engine model can use alternative biodiesel fuels.

Cummins continued production of its 24-valve 5.9L ISB diesel engine until 2007 when it was retired for the 6.7L Cummins, primarily due to increased emissions regulations.

5.9 Cummins Typical Transmissions

The Cummins 5.9L diesel engine could be paired with a manual or automatic transmission.

In the first generation of the engine, the Getrag 5-speed manual transmission was preferred, due to its added extra horsepower. The two automatic transmissions paired with this engine model are the A727 Torqueflite 3-speed Automatic and Chrysler’s 4-speed A518 or 46RH Automatic with overdrive.

The 46RH is more desirable between the two, because it allows for much better fuel economy.

Later model years are paired with Allison transmissions, including AT545, MT643 and the MD3060.

When replacing transmission on either the 5.9L or 6.7L engines, some aftermarket vendors offer a complete kit with an Allison 6-speed automatic transmission.

Towing with a 5.9 Cummins

Towing specifications for the Cummins 5.9L vary depending on cab and model configurations. For example, the tow capacity maximum for earlier models is 11,900 pounds. Check the owner’s manual for the exact towing and payload limitations.

5.9 Cummins Engine Life Expectancy 

The mechanical 5.9L was given a life expectancy of 350,000 miles without catastrophic failure, but some school bus owners will tell you that range is conservative. Owners of a used school bus will likely need to replace some expensive fueling parts long before hitting that kind of high mileage.

The engine turbochargers are considered very durable. In particular, the Holset HX35W found on 1995-1998 12-valve engines is particularly hearty. Aside from the 5.9L Cummins engine itself, the Bosch P7100 (P-pump) is known to last anywhere from 500,000 to 700,000 miles or more before requiring a rebuild. The engine internals – pistons, rod, and crankshaft – can hold up beyond 500,000 miles.

It is also common for the engine to outlive multiple transmissions throughout the course of its life. It’s not so much about the automaker’s inability to build a quality diesel-rated transmission, but rather the engine manufacturer’s I-6 power plant turning out so much low-rpm torque.

5.9 Cummins Engine Maintenance Requirements 

With extended high mileage, owners will need to replace common items. including water pumps, hoses, and belts. Engine longevity is possible by using good quality diesel fuel and conducting proper maintenance of regular oil changes, engine coolant flush, fuel filter replacements, and transmission fluid and filter replacements.

Extra care is needed for engines that operate under severe or heavy-duty conditions, such as excessive idling, dusty environments, frequent hauling, and short trips without reaching full operating temperature. 

5.9 Cummins Problems 

While the Cummins 5.9L diesel has good overall reviews, mechanics have reported issues with components and power. Some school bus mechanics believe that a Cummins 5.9L diesel is too small and underpowered for full-size school buses. Others believe the 24-valve model has more issues due to the additional equipment installed to meet higher emissions standards. Here are some common engine problems:

Lift Pump Failure

The 1998 to 2004 model year 24-valve models commonly experience lift pump failures. The fuel injection system has three primary components: a lift pump, an injection pump, and injectors. With a faulty lift pump, the injection pump may still pull fuel from the back of the gas tank to the fuel injectors, which puts a lot of stress on the system and leads to a breakdown.

Also, early year 24-valve models had the lift pumps attached to the engine block, which caused a lot of excess heat. Failure symptoms include engine misfires, stalling, and rough idling.

In later models (2005+), the lift pump was relocated to inside the gas tank, which improved performance.

Leaking Fuel Injectors

Fuel injectors on the 24-valve model are known to fail around the 150,000-mile mark.

Diesel fuel may contain some dirt particles and other unwanted sediments. If fuel filters are not frequently changed, dirt from the fuel can easily get clogged in the injectors, causing fuel to leak and drip into the engine.

Clogged injectors will not provide enough fuel to each cylinder, which could prevent the engine from starting. Other failure symptoms include fuel in the engine oil and poor idling.

When the vehicle reaches 125,000 miles, it is recommended to check all the injectors for wear and replace them as a set.

Accelerator Pedal Position Sensor Failure

This problem has been known to affect 1998 to 2004 model year engines.

Referred to as APPS, the accelerator pedal position tells the electronic control module (ECM) to open or close the throttle body, which affects the engine’s RMP. When the sensor fails, the ECM doesn’t receive a signal from the pedal, therefore not knowing what to do with engine speeds.

Failure symptoms include a non-responsive pedal and engine surging.

The accelerator pedal can be replaced with an OEM part or an aftermarket replacement.

Exhaust Manifold Leaks

With an inline 6-cylinder and 5.9-liter capacity, the cast iron engine block and exhaust manifold are very long, and one manifold connects them. When heated, the engine block expands. When cooled, it contracts. The constant expansion and contraction cause a lot of stress on the manifold, which cracks and leaks air with the extreme shifts.

Failure symptoms include loss of engine performance, poor idling, engine misfires, and loud noises coming from the manifold.

The only option here is to replace the full manifold quickly to prevent further engine damage.

Engine Block Cracking

Some older 24-valve engines from 1998 to 2002 model years have experienced cracked engine blocks, especially ones from TUPY in Brazil or Teskid in Mexico.

Common causes of block cracking include coolant pressure, corrosion, and frequent towing. Additionally, failing to let the engine properly warm-up prior to running the engine hard can also cause this.

5.9 Cummins Engine Comparisons

5.9 Cummins vs DT466 / DT466E / MaxxForce DT

The Cummins B Series diesel engines (Cummins 5.9L) hit the market in 1984 with agriculture equipment as one of its first uses. By the end of production in 2007, the 5.9 Cummins was a popular engine for pickup trucks, and Types C and D school buses.

The 5.9 Cummins engine was mass-produced for consumer-side RAM medium- and heavy-duty trucks. While it had a close brand association with Dodge pickup trucks, the Cummins engine had the distinction for powering school buses, such as Blue Bird and Thomas Built.

Navistar International’s 7.6-liter six-cylinder diesel turbocharged engine series (DT466/DT466E/MaxxForce DT) had a similar start in agriculture equipment when it was introduced in the early 1970s.

Unlike the Cummins 5.9L diesel engine, the Navistar International DT466 engine series was designed mostly for agriculture, commercial vehicles, and school buses.

Engine Life Expectancy

The DT466/DT466E diesel engines were given a life expectancy from 350,000 to 550,000 miles. Some school bus mechanics claim to have run their DT466-equipped buses for 700,000 miles before requiring an engine rebuild. 

Mechanics report the Cummins 5.9L has enough durability to outlive multiple transmissions throughout the course of its life. With a life expectancy of 350,000 miles, the Cummins engine can go a long way without catastrophic failure. Owners of a used school bus with a Cummins 5.9L will likely need to replace some expensive fueling parts long before hitting that kind of high mileage.

Pros & Cons

Common issues with the Cummins 5.9L tend to be more severe, such as exhaust manifold leaks and engine block cracks. Mechanics report the DT466/DT466E/MaxxForce is easier to fix, especially when considering their all-iron wet sleeve cylinder design. The wet-sleeve design also allows the engine to be rebuilt to factory specifications, sometimes without even removing the engine from the vehicle. However, the DT466E/MaxxForce with its electronic emissions reduction systems to be more finicky than the older DT466 mechanical engines.

5.9 Cummins vs T444E

The Cummins 5.9L (or B Series diesel engines) went into production in 1984 for use in agricultural equipment. Known as the 6BT or 12-valve 5.9L, the engine was a popular alternative to large gasoline V8 engines, such as the Navistar International T444E.

Despite its smaller size, Cummins turbocharged diesel engines could produce significant power with decent torque at low engine speeds. By the end of production in 2007, the Cummins 5.9L had gone through a few iterations for reduced emissions, and it was a popular engine for Types C and D school buses. 

The Cummins 5.9L had strong brand associations with RAM medium- and heavy-duty trucks. A version of the Cummins 5.9L diesel engine was featured in Dodge pickup trucks. The Navistar International T444E had a strong association with Ford F-Series trucks.

Engine Life Expectancy

The mechanical 5.9L was given a life expectancy of 350,000 miles without catastrophic failure, but some school bus owners will tell you that range is conservative. Owners of a used school bus with a Cummins 5.9L will likely need to replace some fueling parts long before hitting that kind of high mileage. Mechanics the Cummins 5.9L has enough durability to outlive multiple transmissions throughout the course of its life. 

Pros & Cons

School bus mechanics report that the Navistar International T444E and Cummins 5.9L are similar in their abilities to sufficiently run small-to-medium size school buses. For more demanding drives, such as bigger school buses and heavier loads, mechanics tend to favor the Navistar International DT466/466E. If maintained properly, mechanics report that the T444E can reach higher mileage over its lifetime than the Cummins 5.9L.

Common issues with the Cummins 5.9L are more severe, such as exhaust manifold leaks and engine block cracks. When it comes to common engine issues, mechanics report the T44E has more simple, inexpensive fixes. 


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