What is a primary cause of excessive blow by in an engine?

Excessive blow-by in an engine primarily occurs due to engine wear, which is the correct answer. As an engine ages and components such as piston rings, cylinder walls, and valves experience wear, it can lead to a loss of the tight seal that is required for proper combustion. This loss of sealing allows combustion gases to escape from the combustion chamber into the crankcase, resulting in blow-by.

While rich and lean conditions can affect engine performance and emissions, they do not directly cause blow-by. A rich condition may lead to incomplete combustion, and a lean condition can create excess heat, but neither condition specifically contributes to the physical degradation of engine components that leads to the excessive blow-by associated with wear. Therefore, the primary cause of excessive blow-by is tied to the deterioration of engine structures rather than fuel mixture imbalances.