blowby

Blowby is the portion of combustion gases that escapes past the piston rings into the crankcase of an internal combustion engine. As the piston moves, some gases leak around the rings during compression and expansion, pressurizing the crankcase and introducing fuel, oil vapor, and combustion byproducts into the lubricant. In most engines a Positive Crankcase Ventilation (PCV) system channels these gases back to the intake to be burned, reducing emissions; in older or non-PCV configurations, blowby gases were vented to atmosphere.

Causes: Blowby increases with ring and bore wear, glazing or scoring, improper ring tension, insufficient lubrication, or high engine speed and load. Poor ring seal allows more gas to flow past rings. A malfunctioning PCV valve or ducting can also cause abnormal crankcase pressure or gas leakage.

Measurement and indicators: Signs include higher oil consumption, oil dilution with fuel, foamy or discolored crankcase oil, and elevated crankcase pressure. Servicing may include PCV valve inspection, crankcase vacuum tests, and, in some cases, measurement of blowby flow. In performance engines, higher blowby can occur under heavy load, but excessive blowby is usually undesirable.

Effects and management: Excessive blowby can dilute oil, promote corrosion, and increase wear on bearings and seals, and raise hydrocarbon emissions. Management focuses on restoring ring seal and bore condition, ensuring a functional PCV system, using an appropriate oil grade, and addressing worn rings or bearings. Significant blowby often indicates the need for engine overhaul or component replacement.