If air is heard coming from the crankcase breather or oil filler during a differential compression check, what does this indicate?

When air is heard coming from the crankcase breather or oil filler during a differential compression check, it indicates that there is a problem related to the sealing of the piston rings. Specifically, this condition suggests that the piston rings are allowing compression to escape into the crankcase.

During a differential compression check, the purpose is to assess the integrity of the engine's combustion chamber. Ideally, air introduced into the cylinder should remain within that chamber, indicating a good seal. However, if the piston rings are worn out or damaged, they will not seal the combustion chamber effectively against the crankcase pressure. As a result, air can leak from the cylinder to the crankcase, which is detected as sound escaping from the breather or oil filler.

This symptom is particularly important because it can lead to reduced engine performance and efficiency, increased oil consumption, and potential further damage if not addressed. Monitoring these indicators during maintenance inspections can help ensure engine reliability and longevity.