Cummins

The Cummins diesel engine — particularly the pre-DEF 5.9L 12-valve — enjoys near-legendary status in the r/BuyItForLife community as one of the most durable engines ever put in a consumer vehicle. Members regularly cite 500,000 to over 1,000,000 miles on these powerplants with routine maintenance only. The primary caveat, repeated consistently, is that Cummins engines often outlive the Dodge/Ram trucks built around them, with the surrounding drivetrain and bodywork considered far less reliable.

The Cummins engine itself — especially the pre-DEF 5.9L 12-valve — is one of the strongest BIFL recommendations in the diesel truck space, but buyers must account for the poor quality of surrounding Dodge/Ram components and the high asking prices for clean used examples.

The community views the Cummins diesel — especially the 12-valve 5.9L — as essentially unkillable with proper maintenance, capable of reaching one million miles and holding strong resale value decades later. It is frequently cited as one of the greatest engines ever made for a consumer truck.

• Pre-DEF 5.9L 12-valve widely considered one of the most reliable engines ever built
• Documented million-mile examples; Cummins has an official Million Mile Club with 120+ members
• Exceptional fuel efficiency relative to other diesel truck options
• Strong resale value; clean examples sell for more than original MSRP
• Mechanically simple design on older variants, easy to work on in the engine bay
• Runs on virtually any combustible fuel without modification on older mechanical injection versions

The engine itself is not the problem — the Dodge/Ram trucks surrounding it are. Community members repeatedly warn that the body, interior, and ancillary components on these trucks are prone to falling apart long before the engine does. Modern post-DEF Cummins engines are also flagged as a liability due to expensive emissions system failures.

• Dodge/Ram truck bodies and interiors are widely considered low quality and rust-prone
• Modern emissions equipment (DEF, EGR) can fail around 175k miles costing $12k+
• Known Killer Dowel Pin (KDP) issue on early 12-valve engines must be addressed proactively
• Pre-DEF trucks are now expensive and hard to find in clean condition
• Odometer maxes out at 999,999 km on some models and requires a computer reset

One commenter put it bluntly: the Dodge didn't last 475,000 miles — the Cummins did, dragging the Chrysler heap along with it kicking and screaming.

The 12-valve Cummins is widely called unstoppable, but the truck interior around it will break if you look at it wrong.

A user noted that pre-DEF diesels like the 5.9 Cummins raise a real sustainability question: isn't a truck that reaches 900,000 miles without emission equipment more sustainable than one that needs $12,000 in DEF system repairs at 175,000 miles?

One owner described a 1998 Ram 3500 with the 5.9L 12-valve as an absolute legendary beast, noting that only the fifth gear walking off and a handful of minor issues came up over the truck's life.