Cummins 5.9L vs. 6.7L The Complete Owner's & Buyer's Guide

Cummins 5.9L vs. 6.7L: The Complete Owner's & Buyer's Guide

By Nel V — The Motor Lab

Buying a used Cummins diesel is a bet. Get it right, and you've got a truck that can run half a million miles. Get it wrong, and you've bought yourself a $6,000 fuel pump replacement with your name on it.

"This guide breaks down exactly how to tell the difference — in plain English, no wrench required."

Here's the simple version of what's inside:

Think of the high-pressure fuel pump as the heart of a common-rail diesel. If it fails, it doesn't just stop — it can send metal "shrapnel" through the fuel system's bloodstream, destroying every injector downstream. We show you exactly how to check for that, on both engines, before you hand over a dime.

Think of the DPF (diesel particulate filter) like a coffee filter for the exhaust. Drive it too gently for too long, and it clogs — just like a filter that never gets rinsed. We show you how to spot that too.

And a "ballooned" crankcase filter? That's the engine literally showing you it's under too much internal pressure — like a balloon about to pop. You'll know exactly what that looks like and what it means.

What you get:

• A full head-to-head on performance, longevity, and build quality between the legendary 5.9L and the modern 6.7L
• A complete, question-by-question inspection checklist covering the fuel system, engine wear, idle hours, and emissions history — answered separately for each engine
• A plain-English buyer's verdict so you know which engine actually fits how you drive and tow
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Who it's for: diesel technicians who want a quick-reference field guide, and diesel enthusiasts who want to buy (or sell) with total confidence.

No guesswork Just the questions that actually separate a great Cummins from an expensive mistake.