Running a compression test taught me more than any spec sheet ever did

Last month I had a 2008 Freightliner with a miss that was driving me nuts, so I finally pulled the valve covers and did a proper compression test instead of guessing. The readings came back at 310, 305, 290, 180 on cylinders 3, 6, 8, and 9, which totally changed how I looked at the whole engine. I found out from a old manual at the shop that these Detroit 60 series should stay within 15% of each other, and that 180 was way outside the limit, so it saved me from rebuilding the whole thing when it was really just a cracked piston head. The numbers told me exactly where to start tearing down, no smoke and mirrors needed. Has anyone else had a compression test reveal something that contradicted what the computer was saying?

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spencer_wood18d ago

Numbers told me exactly where to start tearing down" that part hit home. It's like when my buddy's check engine light was lying to him, but measuring the actual fuel trims showed the real issue. Data over vibes, every time.