Old fleet manager told me to never trust a compression test on a cold engine and I argued with him for 10 minutes before he proved me wrong

He pulled me aside after a job in Des Moines, showed me his gauge reading 380 psi on a warm 5.9 Cummins versus 290 cold, then asked me to explain the 90 pound difference, and I couldn't say a word... has anyone else had a seasoned guy humiliate them with basic thermodynamics like that?

3 comments

3 Comments

the_anthony10d agoMost Upvoted

The part about the dipstick tube blew my mind. Never thought of fuel bleeding past cold rings as a sign of low compression like that. That's a whole different level of old-school diagnostics.

anna49110d ago

Got put in my place by my dad's old buddy back when I was green... he pulled a dipstick tube on a cold 7.3 Powerstroke, let me watch fuel drip out, then said "that's your compression test right there son." Showed me how the rings don't seal right until everything expands... made me sit there and calculate the thermal expansion in my head until I got it. Felt like an idiot for about a week but I never ran a cold compression test again without remembering that lesson.

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aaron_mitchell10d ago

55 degrees. That's what my old shop teacher made me memorize as the minimum temp for even doing a compression check on a small block Chevy. He had this whole spiel about how the piston skirts are basically shrink-wrapped to the cylinder walls when it's cold, and if you try to crank it you're just fighting the metal's natural physics. I actually stuck a thermometer in the exhaust manifold once, waiting for it to hit temp. Felt pretty dumb when he walked by and said "you know that's not measuring the block, right?