For anyone eyeing a new or used diesel truck, a major question looms: will the engine be reliable. Deciding between a Power Stroke, Cummins, or Duramax often comes down to anticipated lifespan and repair frequency. To shed some light on this matter, the owner of PSC Automotive and Diesel Repair, known through the Powerstroke Central channel, compiled a year’s worth of data from his repair shop to reveal which are the most unreliable diesel engines diesels, which trucks visit his shop the most, and why.

Working at Powerstroke Central Automotive and Diesel Repair, the owner acknowledges that his business’s focus skews his numbers toward Ford trucks, but he found trends over nearly a year that offer valuable insights.

“As a diesel repair shop owner, I’ve seen my fair share of trucks come in and out of the shop over the past year,” he stated, explaining that his process involved tracking each truck’s model and needed repairs in a spreadsheet.

Common diesel engines that come to his shop for some repairs

Power Stroke Engines

The owner shared his findings when looking at specific engines. The 7.3-liter Power Stroke stands out, often seen with over 300,000 miles, typically needing age-related fixes like injectors or wiring. “That is why I’m putting the 7.3 as most reliable because they are just the most high-mile trucks,” he said.

While the 6.0-liter Power Stroke is known for significant issues like high-pressure oil system failures and head gaskets, he notes they can be made dependable with proper upkeep.

In stark contrast, the 6.4-liter Power Stroke is identified as the most unreliable in his experience. “Out of the 8 6.4 Power Strokes that we’ve seen, almost every single one of them had catastrophic engine failure.” he reported, adding, “Stay away from the 6.4 Power Stroke.”

Duramax Engines

For Duramax engines seen at his shop, the owner points to the LML Duramax (2011-2016) also as unreliable, citing frequent DPF (Diesel Particulate Filter), EGR (Exhaust Gas Recirculation), and fuel system problems.

“Stay far far away from the LMLs if you guys can,” he advised, noting, “These trucks fail; when the LMLs fail they fail badly.” Other Duramax years seem less prone to visiting his shop, suggesting better reliability in his view.

Cummins Engines

Among Cummins trucks, the 5.9-liter Cummins shows extreme reliability, often needing only maintenance or age-related injector work, earning it a spot near the top with the 7.3-liter Power Stroke. The 2013-2018 6.7-liter Cummins models are also very reliable, typically seen for minor issues at high-mileage.

However, based on his observations, he finds the 2018-2021 6.7-liter Cummins with the CP4 high-pressure fuel pump and lifter issues to be unreliable. “Those have got to be one of the most unreliable that we have seen,” he commented.

Longevity Tip

The shop owner stresses a key factor in longevity: maintenance. “If you guys actually want to see 400,000-500,000 miles out of your diesel truck, you need to maintain it properly,” he stated. “90 percent of the customers that come in here that have failures did not do their maintenance.”

He also concluded: “When it comes to overall reliability, I would say the 7.3 and the 5.9 Cummins are the most reliable diesel trucks that have ever been made to date.”

Thoughts on the Most Unreliable Diesel Engines?

This perspective about the most unreliable diesel engines comes directly from Powerstroke Central Automotive and Diesel Repair. Though some fans are arguing which is which, diesel engines don’t need to prove anything else. What are your thoughts on these reliability takes? Have your experiences with these diesel engines been similar or different?