While the second-generation Dodge Ram Cummins 5.9-liter turbo-diesel engines are at least 19 years old, they are still known to be very reliable. Produced from 1994 through 2002, the overhead valve inline-six engines feature cast iron engine blocks and cylinder heads, cast aluminum pistons, direct fuel injection, and a single Holset turbocharger capable of holding stock boost pressure.
However, when trying to decide between the 12- and 24-valve versions of the engine, it comes down to what you are hoping to accomplish with your light-duty truck as there is no clear-cut winner between the two. Let’s take a look at how the two versions compare to each other.
The 12-valve 5.9 Cummins is also known as the 6BT engine. Compared to the first generation of 5.9-liter engines, this generation had better emissions and performance thanks to a new turbocharger, intercooler, injectors, and pistons.
With a single Holset HX35W turbocharger, the earlier 6BT produced 160-175 horsepower and 400-420 pounds-feet of torque, depending on the transmission (manual transmissions had higher output numbers). From 1996 through 1998, output was increased to 180-215 horsepower and 420-440 pounds-feet of torque.
The 12V Cummins offers minimal electronics, high horsepower potential, and overall simplicity.
Its Bosch P7100 fuel injection pump, sometimes called the “P-pump,” can be easily modified for higher flow and better performance as almost all of its parts can be upgraded. With a plunger for each cylinder, the P-pump was installed to improve the engine’s emissions by decreasing the amount of particulate matter in the cylinders. In addition, the engine’s fuel injectors were designed to endure higher fuel pressures.
One of the biggest issues with the 12V Cummins is a small dowel pin located in the front of the engine. Installed during assembly, the “killer dowel pin” helped with aligning the timing cover. The engine’s vibrations can cause the dowel pin to come out over time and potentially end up behind the front engine cover and between timing gears, causing damage.
The 12-valve 5.9 Cummins also can have issues with the injection pump’s overflow valve, which can leak or end up with a broken spring or an eroded seat. The throttle position sensor can also fail, resulting in unbalanced air-to-fuel ratios.
The 24-valve 5.9-liter Cummins turbo-diesel engine, otherwise known as the ISB engine, offers higher stock performance numbers and the ability to tune the engine via a handheld tuner.
Just like the 12V engine, the 24V 5.9 had a single Holset HX35W turbocharger through the 1999 model year. Beginning in 2000, the engine used a Holset HY35W turbo, which had a more restrictive exhaust setup. The 1998 through 2000 engine produced 215-235 horsepower and 420-460 pounds-feet of torque, once again with the manual transmission offering higher numbers. For 2001 and 2002, the 24V’s figures increased to 235-245 horsepower and 460-505 pounds-feet of torque.
Cummins also produced a high output version of the 24V engine. Paired with a New Venture NV5600 six-speed manual transmission, the high output 24V 5.9 made 10 more horsepower and 45 more pound-feet of torque over the standard version.
With better throttle response and drivability, the 24-valve engine has better airflow and coolant flow compared to the 12-valve version. The engine also features a reusable gasket for the one-piece valve cover, an improvement over the 12V’s six individual covers.
Instead of a mechanical system, the 24Vs fuel injection pump is electronically controlled and can be easily tuned with a tuner. Working with the truck’s engine control unit, the rotary Bosch VP44 injection pump efficiently controls fuel flow and injection timing. The 24V also has an electric fuel lift pump to help provide consistent fuel pressure.
A major downside of the 24-valve Cummins 5.9 was the thinner walls of the cast iron engine block. Specific to the earlier ISB engines (1999 through 2001) stamped with the number “53,” the thinner blocks were susceptible to cracking by the freeze plugs, resulting in coolant leaks.
Additionally, while the Bosch VP44 injection pump is a strong point, it is also one of the more common issues of the 24V engine and can be expensive to repair. The fuel lift pump can also fail. Other problematic parts include the engine’s exhaust manifold, which can crack or leak, and fuel injectors.
As mentioned earlier, there is no right or wrong answer as to which version of the 5.9 Cummins is best. Even though both engines are known for their reliability, they are now more than 19 years old, which means they will require some maintenance and repairs.
Those who want high-performance numbers may prefer the 12V version as it can be built to exceed 1,000 horsepower with small upgrades. On the other end of the spectrum, the newer 24V may be more ideal for those who are hoping to make upgrades using a handheld tuner or who want to keep their truck more stock for the street.
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