Too much crankcase blow-by is most likely caused by which of the following?

Too much crankcase blow-by is primarily caused by worn piston rings. When piston rings wear down, they can no longer effectively seal the combustion chamber during the compression and power strokes of the engine. This allows combustion gases to escape past the rings into the crankcase, resulting in increased blow-by.

The condition of the piston rings is critical because they are designed to maintain a tight seal between the piston and the cylinder wall, thereby preventing excessive amounts of fuel and combustion gases from entering the crankcase. When this seal is compromised due to wear, it leads to an increase in pressure and gas volume in the crankcase, which is referred to as blow-by.

In contrast, issues such as a leaky exhaust manifold or worn exhaust valve guides typically do not directly contribute to crankcase blow-by, as they involve different components and systems of the engine. A leaky exhaust manifold would affect exhaust gas flow rather than cause an issue with the sealing of the combustion chamber, while worn valve guides primarily pertain to the movement and sealing of the valves. Thus, the presence of worn piston rings is the most significant factor concerning excessive crankcase blow-by.