These Are Some Of The Worst Cummins Engines Of All Time, According To Owners

You don’t have to be a diesel-head to have heard about Cummins engines. The Indiana-based diesel truck engine manufacturer has outdone itself in developing some of the strongest and reliable engines in the market. Cummins engines power a wide range of applications, from heavy-duty pickups like the Ram 2500 and 3500 HD, the Nissan Titan XD, and Isuzu trucks, to commercial vehicles and industrial machinery.

The best years for Cummins diesel engines are a highly debated topic among diesel enthusiasts. Most fans regard the 12-valve 5.9L 6BT (introduced in 1989) as the engine that put the Dodge Ram on the diesel map. Its simplicity, reliability, and durability have earned it a cult-like following. But not all engines are made equal. Cummins has dropped the ball a couple of times developing diesel engines, and at the top of its lemon list is the 5.0L Cummins V8 powering the discontinued Nissan Titan XD. The 5.9L (24-valve) code-named “53-block” was notorious for a cracking block. Meanwhile, the early Cummins ISX inline six had issues with the camshafts caused by faulty rocker arms, as well as EGR system malfunctions.

5.0L Cummins V8 on 2016-2019 Nissan Titan XD trucks

The Cummins 5.0L V8 that powered the Nissan Titan XD was designed to be groundbreaking, leveraging Nissan’s experience developing innovative, safe, fuel-efficient, and reliable vehicles, and Cummins’ solid reputation as a powerhouse in the diesel engine industry. The product of their partnership, the Titan XD, aimed to bridge the gap between the half-ton and three-quarter-ton classes of trucks. Unfortunately, it didn’t take long after the release of the Nissan Titan XD in 2016 for owners to start complaining of problems with the 5.0L Cummins V8 engine. After a brief production stint, Nissan discontinued the Titan XD in 2019.

Some of the commonly reported problems with the Cummins V8 engine in the Nissan Titan included a DEF system malfunction that may be caused by a failed DEF pump. According to a 2016 Nissan Titan XD owner on a popular mechanics forum, the repair for the malfunction caused by a failed pump is quite pricey, and they were quoted $4,800 by their Nissan dealership for a DEF system part replacement.

A 2017 Titan XD owner on the Titan XD Forum also reported experiencing turbocharger issues, which resulted in a loss of power, dashboard codes, and hard shifting and jerking during towing. In 2022, Nissan (PDF) released a campaign bulletin for an “Emission Warranty Extension” covering 14,269 units, stating that specific Titan XD models may experience issues with the turbo actuator. Owners have also reported EGR valve, cooler, and fuel system issues on the 5.0L V8.

1998-2002 Cummins 5.9L ISB 24-valve

The 5.9L engine was an inline six-cylinder engine developed for heavy-duty truck applications. The “ISB” stood for “Interact System,” while the “B” meant it was part of the Cummins B-series of engines. This engine was an improvement over its predecessor, but it also featured some issues. A select line of Cummins 5.9L ISB 24-valve engines — developed between 1999 and 2002 for Ram trucks with a 53 Block casting — were prone to block cracks due to a thin crankcase around the water jackets, resulting in coolant leaks.

This 5.9L Cummins ISB engine also suffered from lift pump failure, which led to fuel system issues, particularly with its fuel injectors and a high-pressure Bosch VP44 injection pump (VP), resulting in fuel starvation and power loss. This was a common repair for owners between 1998 and 2002. “Some of the signs of a dying lift pump. You see a boost dropping for no reason. You notice the engine kind of laying down in the upper RPM ranges,” an owner confessed on Cummins Forums.

Early Cummins ISX

The Cummins ISX was introduced to meet the stringent emission standards that came at the turn of the century. It replaced the N14 Cummins (based on the 855 Big Cam) and was primarily used in the Class 8 trucks, as well as industrial, agricultural, and marine applications. It featured some of the best aspects of its predecessor, adding improved fuel efficiency, reduced emissions, and increased power. It was a good move for Cummins, stepping into the modern world. Still, the Indiana-based diesel engine maker experienced some issues, specifically with systems designed to improve upon its predecessor, the N14. Some of the most common issues among ISX Cummins owners were related to the EGR (exhaust gas recirculation) valve, cooler, and sensor failure.

The Cummins ISX is also known to have issues with the rocker arms that can lead to a loss in performance or, worse, camshaft damage. According to owners, the leading cause of this is overheating, which warps the cylinder head, causing the camshafts to operate in a crooked bore. This damages the bearings and, in some instances, breaks the rocker arms.

2013-2019 Cummins 6.7L ISB

The Cummins 6.7L ISB debuted in 2007, midway through Dodge’s ’07 model year production of Ram heavy-duty pickup trucks. It’s featured among the most powerful diesel engines developed at that time for pickup trucks, boasting a towing capability of up to 31,000 pounds (when mated with the AsinAS69RC) on the Ram 3500. The Cummins 6.7L ISB was a fan favorite due to its tunability and ability to accommodate aftermarket performance modifications.

The 6.7L turbo diesel engine featured a VGT (Variable Geometry Turbocharger) designed to reduce lag, incorporate exhaust braking, and provide infinite adjustment to the pickup truck’s load demand. However, the VGT system had some issues, either with a failed actuator or stuck vanes inside the turbo, resulting in problems with the exhaust brake. Although it was powerful, it had emission tech issues with the EGR system. Turbo actuators seem to be a big issue with the 6.7L Cummins. An owner from a popular diesel forum with a 2016 6.7L Cummins confessed that their truck experienced the same issue with just 16,000 miles. Another owner, with a 2014 engine, disclosed that their turbo actuator was the only component to fail before reaching 161,000 miles.

The way forward for owning a Cummins-powered truck

It’s not uncommon for great brands to produce duds. Toyota, known for its reliability, has at least four unreliable engines, and Subaru, which held a top spot in the 2025 Consumer Reports, received poor ratings (below average reliability) for the Solterra EV. Cummins, like any other auto manufacturer, has had its fair share of problems with some of its engines. Still, the diesel engine manufacturer has built a good reputation for developing some of the most reliable engines, which can easily reach up to a million miles (Cummins Million Mile Club) with proper maintenance.

A rule of thumb for buying a truck with a Cummins engine is to always check the vehicle identification number (VIN) for any recalls, technical service bulletins (TSBs), or known issues. It is also a good idea to join popular diesel forums, such as the Cummins Forum and the Cummins subreddit on Reddit, to gain real-world insights from owners on maintenance and preventive care. Lastly, joining a local community of diesel truck lovers will make a big difference in how well you understand your truck.