Blow-by in a diesel engine is when a mixture of air-fuel or combustion gases leaks between the engine’s pistons and cylinder wall into the crankcase. It occurs when the internal combustion that occurs in the engine’s combustion chamber forces contaminants such as air, fuel, and moisture past the piston rings and into the crankcase. Blow-by can happen within both diesel and gas engines. Blow-by affects important engine processes and can cause the following symptoms:

• Rough idling or misfiring: If the engine is difficult to start or keep running, blow-by might be the culprit, as blow-by affects important engine processes.
• Loss of compression: Blow-by inhibits overall engine performance due to a loss of compression.

Blow-by can be caused by several factors, including marginal maintenance, bad or broken piston rings, defective or damaged pistons, and gaps between the piston rings. Blow-by can seriously harm the engine’s fuel economy, performance, and effectiveness, depleting engine power and increasing gas pressure in the crankcase. It is a common occurrence in diesel engines. Regular maintenance and checking the state of ring seal in the engine can help prevent excessive blow-by.