Common Problems With The 5.9 Cummins, According To Owners

Cummins diesel engines and the Ram truck combined forces some years ago, creating what some would describe as the best partnership since peanut butter and jelly. The first-gen Cummins engines cost a pretty penny today, and it's because these 5.9-liter diesels were fantastic, along with being so influential to the industry.

Just how much torque does a 5.9 Cummins engine have? The specifications were impressive back in the late 80s and only improved to the point that when the 5.9 Cummins was replaced in 2007 with the 6.7-liter, it offered an additional 210 pound-feet of torque over its predecessors. In fact, fans say these iterations are among the best years for Cummins diesel engines.

Naturally, no engine is perfect, and while revered, the 5.9-liter Cummins does have some notable weaknesses, as described by owners. Examples of widely reported problems include the killer dowel pin and injector failures.

Found in the 5.9-liter Cummins turbodiesel engines is a small, cylindrical metal part that functions to align the timing gear housing with the engine. While this dowel seems innocuous, it poses a potentially catastrophic threat to the health of your engine. In some cases, this little metallic piece can dislodge and cause one of two scenarios: it can fly into the timing case, wreaking havoc and causing damage to multiple components, or it can cause an oil leak.

One owner explained, "Most of the time [the killer dowel pin] wrecks the front timing case when it goes between a gear and the aluminum case. Inspection of pump, cam, crank, oil pump gear is recommended." A few owners chimed in, explaining they experienced the dowel issue, but it didn't cause any damage other than an oil leak, though one response comically suggested they should consider playing the lottery. Fortunately, the killer dowel pin on the Cummins only affects certain years, and there are low-cost, do-it-yourself solutions to prevent the issue from happening.

Diesel fuel injectors are a vital component to a healthy engine and make Cummins' impressive output figures possible. However, common symptoms of diesel fuel injector failure include rough idling and lackluster power, to the point that you'll be lucky to limp the truck back home. One owner of a 5.9-liter Cummins was driving when a few issues occurred in rapid succession: the check engine light illuminated, output was suddenly weak, and the engine seemed to be struggling to run properly. It turned out that the #2 injector was the cause of the issue, with other owners echoing similar experiences.

It isn't always just one injector causing the problem, either, but multiple ones. For example, an owner experienced sudden vibration and odd engine sounds while on the freeway, and their mechanic discovered that 2, 4, and 6 injectors weren't functioning properly. Another reported problem with the injectors in the 5.9 Cummins is leaking, with signs of a leaking injector including things like white smoke and a strong diesel fuel smell. One owner explained, "[There are] three places [an injector] can leak, return banjo, tube nut or nozzle seat."

Regardless of which generation 5.9 Cummins you own, it's a good practice to know how to clean fuel injectors and keep your engine running optimally.