What does diesel engine blow-by mean?

Blow-by is combustion pressure escaping past the piston rings into the crankcase. Some crankcase vapor is normal on many diesel engines, especially with age, but excessive pressure can point to engine wear or other problems. A quick oil-cap check can be useful, but it is not a complete engine diagnosis. Look at the whole picture: cold start behavior, smoke, oil consumption, crankcase pressure, leaks, idle quality, service records, and whether the truck pulls cleanly under load. If blow-by looks heavy, the oil cap dances aggressively, or the seller dismisses obvious symptoms, get a professional inspection or move on.