During which operation does excessive pressure in the crankcase most commonly occur?

Excessive pressure in the crankcase most commonly occurs during startup due to the rapid increase in oil circulation and the slower evacuation of air within the crankcase. When an engine starts, oil is circulated throughout the system to lubricate the moving parts. However, at startup, the pistons are still at rest, and the crankcase has not fully vented yet. This can lead to a brief moment where the crankcase pressure rises higher than normal until the engine reaches an operational speed where ventilation becomes more effective.

Additionally, during startup, the engine is often cold, and oil is more viscous. This can further contribute to pressure buildup as the oil moves more sluggishly through the system. As the engine warms up and reaches its normal operating RPM, the pressure typically stabilizes as the crankcase ventilation and oil circulation improve, helping to mitigate any excessive pressures.

In contrast, at idle, the engine generally maintains a stable and lower pressure, and during high RPM operation, the increased airflow and oil circulation typically help to keep crankcase pressures in check, preventing excessive buildup.