Unkillable Diesels: 5.9L Cummins

• November 20, 2020
• Story By
  Mike McGlothlin

It would be wrong, on a whole host of levels, to exclude the B-series 5.9L Cummins from any meaningful conversation about bulletproof engines. In fact, with the ability to last a million miles in a truck or tens of thousands of hours in a piece of equipment, the 359ci inline-six epitomizes the term unkillable. Its design is simple where it needs to be, robust where it matters most and a well-maintained version always yields decades of loyal operation. Almost insultingly, the mechanical 5.9L was given a B50 life of 350,000 miles, which every Cummins owner will tell you is extremely conservative.

When you learn about the nuts and bolts of this legendary power plant, you begin to understand why it’s a forever engine. We’ve got all of that and more below.

Author’s Note: For our purposes, we’re talking specifically about the P-pumped 12-valve version of the 5.9L Cummins, the one that was offered in Dodge Ram 2500 and 3500’s from 1994-1998. However, much of this information applies to the 1989-1993 engine, too.

I-6 Architecture and A Beefy Bottom-End

The bulletproof nature of the 5.9L Cummins begins with its inline-six design, an inherently balanced configuration. Add in its 125-pound, forged-steel crankshaft, the seven main bearings that support it and the sizeable 14mm diameter main cap bolts that secure it within the cast-iron block and you start to see why the bottom end of the 6BT is so stout. Each forged-steel I-beam connecting rod enjoys its own rod bearing journal and is separated by a main bearing journal, all of which were treated to induction hardening (for improved strength and fatigue life). Rod bolts measuring 7/16-inches secure the rod caps to the crank.

The Tough-As-Nails 12-Valve Head

A simple, cross-flow cylinder head cast from gray iron, the 6BT 5.9L employs just one intake valve and one exhaust valve per cylinder. All 12 valve seats were induction-hardened and both the intake and exhaust valve rockers were made of ductile iron rather than stamped steel. For optimum combustion sealing, there are six head bolts per cylinder, with one fastener per cylinder asked to multi-task in securing the rocker lever assembly in place. Thanks to the 5.9L Cummins’ six bolt per cylinder arrangement, head gaskets are never an issue—not even with 45 psi or more of boost in the mix.

Over-Engineered + Low Horsepower = Decades of Longevity

Overbuilt and a lack of power means infinite operation for most diesels—and the 5.9L Cummins is no exception. When spec’d for construction or agricultural equipment, some engines were detuned to as low as 90 hp. Compare that with the 160 hp to 215 hp versions found in Dodge Ram 2500 and 3500 trucks, which were still capable of lasting a million miles, and it makes you wonder exactly how long a 5.9L making half that horsepower could last. Imagine scoring a 30-year-old engine on eBay that doesn’t need to be overhauled and might not ever have to be!

The P-pump Lasts Forever, Too

Aside from the 5.9L Cummins engine itself, the Bosch P7100 (a.k.a. P-pump) is known to last anywhere from 500,000 to 700,000 miles or more before requiring a rebuild. Of course, this type of near-eternal service is only possible with proper maintenance that consists of regular fuel filter changes and burning good quality diesel fuel, but there is no secret to P-pump longevity. The old-school, fully-mechanical plunger pump is fairly complicated, but like the Cummins power plant it’s bolted to its internal components (cam, plungers, barrels, delivery valves, delivery valve springs) rarely ever fail.

Long-Lasting Injectors

Completely mechanical, the injectors in the P-pumped Cummins open according to a set pop-off pressure. The pressure they see is supplied to them via the P-pump and in the ’94-’98 5.9L’s case, pop-off pressure checked in at 260 bar (3,770 psi) from the factory. In addition to being 100-percent mechanical (i.e. no electronic solenoid or computer telling them what to do), the injectors are extremely simple, with very few moving parts. An owner can expect to get anywhere from 250,000 to 1 million miles out of a set, depending on the type of care the fuel system is treated to.

Holset HX35W: Built Like A B-Series

Like the 6BT Cummins, the turbochargers bolted to them are hard to kill. In particular, the Holset HX35W found on ’95-’98 12-valve engines is particularly bulletproof. This turbo made less than 20 psi of boost in stock form, but can be subjected to 40 psi on a regular basis and survive without overspeeding. Trust us, a lot of other turbochargers check out when asked to turn twice the rpm and build double the boost and drive pressure they were originally intended to see. Not surprisingly, Holset is a branch of Cummins and has been since 1973, hence the overbuilt nature of these turbos.

Another Key To Longevity: Lack of Emissions Equipment

It could be argued that the 5.9L Cummins’ ability to last ‘till Doomsday stems from its lack of emissions equipment. And in all honesty, the same can be said for virtually any diesel engine that was void of today’s EGR, DPF and SCR systems. Other than the presence of a catalytic converter on ’94.5-’98 federal engines, not one component on the 5.9L could stifle performance, hinder longevity or fail prematurely. The one exception to this is ’96-’98 California model engines, which were fitted with EGR in order to meet CARB’s more stringent NOx standards of the late ‘90s. In case you’re wondering, these rare 5.9L Cummins mills did not respond well to EGR.

A Cummins Is Forever—Its Transmission Is Not

It’s a pretty well-documented fact that the 5.9L Cummins will outlive the Dodge truck it’s in, but it’s also common for it to outlive multiple transmissions throughout the course of its life. In fact, at power levels even barely beyond stock, the 5.9L Cummins (of any lineage from ’89 to ‘07) has always been able to push Chrysler’s automatic slushboxes over the edge. 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. Low-end grunt is the biggest killer of any diesel transmission.