Engine Blow-by Engine blow-by is a condition that results from engine combustion gases blowing past a blown piston ring and entering the engine head area, where these combustion gases can be visibly seen exiting the engine oil crankcase filler hole or even the carburetor throttle body opening. Engine blow-by is routinely noted during a normal vehicle engine tune-up or some other type of routine vehicle maintenance. Rough Engine Idle Due to the engine combustion gases that escape past a blown piston ring, it is common for a vehicle engine with a blown piston ring to idle roughly or erratically. This rough engine idle is due to the loss of engine compression that results from the blown piston ring. The more engine cylinders that have a blown piston ring, the rougher the engine idle will be.