Written By, Mike McGlothlin
Highlights:
Head-to-Head Comparison: Breaks down the strengths and weaknesses of the 12-valve (6BT) and 24-valve (ISB) 5.9L Cummins engines
Shared Traits: Both engines boast durable architecture, high-performance potential, and common issues like the killer dowel pin and rear cylinder heat concerns
12-Valve Advantages: Fully mechanical fuel system, P-pump reliability, million-mile durability, and high resale value
24-Valve Advantages: Higher stock horsepower/torque, improved airflow, cleaner emissions, and longer valve adjustment intervals
Key Drawbacks: 12-valve has few quirks (like overflow valve issues), while the 24-valve faces bigger concerns such as VP44 pump failures, weak lift pumps, “53 block” cracking, and APPS sensor failures
5.9L vs. 5.9L. Cummins vs. Cummins. 6BT vs. ISB. 12-valve vs. 24-valve Cummins. It’s a head-to-head matchup between legendary inline-six diesel engines that deserves exploring. After all, either generation of 5.9L Cummins has its strong suits and weak points—along with a highly loyal following backing either rendition. Beyond their bulletproof hard-part architectures, the 12-valve and 24-valve both share common pitfalls while also suffering from their own specific sets of issues. Below, we’ll spotlight their best attributes—and then turn the page to expose their greatest shortcomings. From there, we’ll let you decide which 5.9L is the best Cummins engine.
Shared Advantages
As the 12-valve 6BT’s successor, it makes perfect sense that the 24-valve ISB shares some
of its DNA with the O.G. 5.9L Cummins. Among the similarities, both engines feature thickly cast, gray-iron block and head construction, utilize six head fasteners per cylinder for optimum clamping force and head gasket life, and use the same exact connecting rods. The epitome of robust, the forged-steel, I-beam rods (employed from ’89-’02) are capable of surviving more than 1,000 horsepower and 1,500 lb-ft of torque. As a bonus, both the 6BT and ISB were equipped with the same, virtually indestructible Holset HX35W turbocharger at one point. Last but not least, both engine platforms can support 500 hp with few, if any, required hard part changes.
Shared Disadvantages
The list of common drawbacks is short between the 6BT and ISB, but it exists
nevertheless. First and foremost, the killer dowel pin is present in every 6BT Cummins as well as most 24-valve engines. The killer dowel pin (KDP) can strike when the 5/16-inch diameter steel dowel pin—used to align the front cover to the engine block on the assembly line—works itself loose due to vibration, heat cycles, or age. If it falls into the camshaft gear, it can inflict piston-to-valve contact and essentially destroy the engine. Another common problem across both engine lines is their tendency for cylinders 5 and especially 6 to run warm. This is the result of poor coolant flow to the rearmost cylinders which, over time, can result in overheating, head gasket, freeze plug, and even piston ring issues.
12-Valve Pros
Aside from the inherent longevity that comes with an industrial strength, all-iron inline-six chock-full of forged-steel internals, the 12-valve’s biggest point of durability is its use of a fully mechanical injection system. From the cam-driven fuel supply pump (i.e. lift pump) to the injection pump, and on to the pop-off style injectors, the 6BT Cummins’ fueling is fully mechanical. That means no wires or electronic modules to fail, and primitive yet simplistic operation. Specifically, 6BT’s equipped with the Bosch P7100 injection pump (found on ’94-’98 Ram engines) are the most coveted. Million-mile durability, near-endless performance potential (which could be unlocked via wrenches), and high resale value arguably make this version of the 5.9L the most desirable ever built.
24-Valve Pro: More Powerful (Stock Vs. Stock)
Pound-for-pound, the ISB 24-valve produced more horsepower and torque in factory
form. Introduced midway through the 1998 model year for Ram trucks, the 24-valve Cummins packed 235 hp and 460 lb-ft of torque right out of the box when equipped with a five-speed manual transmission option. In comparison, the most powerful OEM form of the outgoing 12-valve engine produced 215 hp and 440 lb-ft of torque (again, when fitted with the five-speed manual option). In 2001, a high output version of the ISB 24-valve was launched and made available in conjunction with a six-speed manual transmission. It produced 245 hp and 505 lb-ft of torque.
24-Valve Pro: Improved Airflow & Cleaner Emissions
Packing four valves per cylinder vs. two, it stands to reason that the 24-valve ISB
Cummins’ cylinder head breathed a bit better than its 12-valve predecessor—especially at higher rpm. But that wasn’t all the 24-valve head provided. It would also boast a significantly longer recommended valve adjustment interval than the 12-valve (150,000 miles vs. every 24,000 miles or 24 months). And although that 150,000-mile recommendation would prove a bit generous, most 24-valve ISB engines could go 90,000 to 100,000 miles before it was time to run the valves. Additionally, the 24-valve Cummins’ redesigned pistons, a cylinder head which directly centered the injectors over the pistons, and higher-pressure fuel injectors all contributed to lower tailpipe emissions.
12-Valve Cons : Very Few
Beyond the killer dowel pin issue, there are no major points of concern specific to the 12-valve Cummins. That said, there are a few quirks on ’94-’98 engines, the biggest of which is the overflow valve located within the Bosch P7100. A poorly designed factory ball and seat places added stress on the internal OEM spring, which noticeably reduces fuel pressure inside the P7100 and can even eventually break the spring. Immediate signs of overflow valve issues are hard-starting, reduced performance, and increased smoke levels. Further down the severity scale, leaking individual valve covers has been a longtime gripe of 12-valve owners. Age and vibration are the common culprits there.
24-Valve Con: Electronic-Over-Mechanical Injection System
While the fuel injectors remained mechanical on the 24-valve ISB Cummins, electronic
control did enter the picture in 1998—and it came in the form of the VP44 injection pump. The distributor style Bosch pump was tasked with metering fuel independently from engine speed and it was accomplished through the use of an integrated computer (called the PSG) married to the top of the VP44. Unfortunately, the PSG was prone to premature failure, primarily due to heat. The only way to keep a VP44 and its integrated PSG running cool is to ensure the pump is always being supplied a steady 12-15 psi worth of fuel pressure from the lift pump.
24-Valve Con: Lift Pump Failure
This disadvantage is huge in the world of Cummins engines, and it’s a failure that’s been
well-documented over the years. Although Cummins changed to a self-priming electric lift pump (albeit engine-mounted) on the ISB 5.9L, vibration tended to kill it. This often led to the VP44 being starved for fuel. In an expensive chain reaction of events, the lift pump would take out the injection pump, and many owners went through several VP44’s before Chrysler released its in-tank lift pump solution, which was followed by various aftermarket chassis-mounted pump upgrades. Any time a VP44 checks out (with either type of the aforementioned fuel supply pump feeding it), it pays to ensure the lift pump didn’t cause the failure.
24-Valve Con: “53 Block”
An injection pump and a lift pump can be replaced easy enough—not so much with an engine block. Unfortunately, Brazilian manufacturer TUPY produced roughly 100,000 ISB Cummins blocks, found primarily in ’99-’01 model year Ram trucks, that were cast with very little iron present around the water jackets—and the thinly cast areas were prone to cracking. These blocks wore the infamous “53” casting number on the driver side front of the crankcase. While the thin casting issue was resolved on later blocks (manufactured by Teskid of Mexico), Chrysler had to replace a boatload of engines under warranty because of the cracking. Years down the road, out-of-warranty ISB owners have dabbled in block stitching and/or had to replace the block themselves.
24-Valve Con: APPS Failure
Hey, we told you electronics add complexity, didn’t we? Another highly common issue
with the 24-valve Cummins is its tendency to suffer APPS failure. APPS stands for Accelerator Pedal Position Sensor, a fancy title for what amounts to a throttle position sensor. The APPS provides the 24-valve engine’s ECM with a DC voltage signal, which varies as the position of the accelerator changes. When an APPS starts to go south, it’s because the internal rotating fingers begin losing contact with the resistance track they ride in. Erratic engine behavior like surging, high idling, and “dead pedal” scenarios are key indicators that the APPS is failing, with the latter symptom being the most typical. It’s a frequent failure that has been part of the 24-valve Cummins’ rap sheet for more than 20 years.
