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

When air is heard coming from the crankcase breather or oil filler during a differential compression check, it strongly indicates piston ring leakage. This situation occurs because the differential compression check measures the ability of the engine's cylinders to hold pressure, and if there are leaks present in the piston rings, the compressed air can escape into the crankcase.

Piston rings are designed to create a seal between the piston and the cylinder wall to maintain compression; if they are worn, damaged, or improperly seated, they can fail to contain the pressure, resulting in audible airflow escaping through the crankcase breather or oil filler cap. This leakage is often referred to as "blow-by," where combustion gases escape past the piston rings into the crankcase, which not only affects engine performance but also leads to increased oil contamination.

While valve seat leakage can also cause compression loss, it typically would result in air escaping through the intake or exhaust system, rather than the crankcase. Oil pump failure typically does not produce an audible airflow symptom in relation to a differential compression check, and excess blow-by is a symptom of piston ring issues but is more accurately described through the mechanism of piston ring leakage itself rather than as a standalone issue. Thus, the presence of air