Oil blow by represents a pervasive yet often misunderstood phenomenon within internal combustion engines, where combustion gases bypass the piston rings and escape into the crankcase. This process occurs under the high pressure generated during the power stroke, forcing past the relatively thin oil film that seals the ring pack. While typically a small and manageable part of engine operation, excessive blow by signals underlying issues that can rapidly degrade performance and reliability.
Mechanics of Blow By
The fundamental mechanics involve the complex interaction between the piston rings, cylinder walls, and the viscous oil film coating the bore. During combustion, peak pressures can exceed 2000 psi, creating a desperate need for a seal. The compression rings, acting as primary scrapers and sealers, are designed to maintain this barrier. However, microscopic gaps always exist, particularly at the end gaps of the rings and in the back-rake angle behind the ring face. These pathways, though engineered to be minimal, become conduits for hot, high-pressure gases under constant pressure.
Contributing Factors and Wear
Several factors directly influence the rate and volume of oil blow by. Cylinder bore wall condition is paramount; scoring, honing imperfections, or excessive wear increase the gap between the ring and the metal, allowing a greater volume of gas to pass. Equally critical is the state of the piston rings themselves; ring end gap wear, loss of tension, or incorrect installation angles reduce their sealing efficiency. The quality and temperature of the engine oil also play a significant role, as improper viscosity or degraded additive packages fail to provide adequate lubrication and sealing support.
Consequences of Excessive Oil Blow By
When oil blow by exceeds the capacity of the crankcase ventilation system, a cascade of detrimental effects begins. The most immediate consequence is the contamination of the engine oil itself. Raw fuel vapor and soot particles from combustion mix with the lubricant, diluting its viscosity and neutralizing its additives. This sludge-like mixture loses its ability to lubricate moving parts effectively, leading to increased friction and premature wear on bearings and cylinder walls.
Crankcase Pressure and Seal Failure
Another critical impact is the pressurization of the crankcase. A functioning PCV (Positive Crankcase Ventilation) system relies on a slight vacuum to pull these gases back into the intake manifold for re-burn. Excessive blow by overwhelms this system, resulting in abnormally high crankcase pressure. This pressure forces oil past any weak seal it can find, including valve cover gaskets, oil pan seals, and dipstick tubes, creating visible leaks and environmental messes.
Performance and Emissions Impact
From a performance standpoint, oil blow by directly robs an engine of its power potential. The pressure wave created in the crankcase fights against the downward force of the piston on the exhaust stroke, creating what is effectively a parasitic load. Furthermore, the blow-by gases contain unburned hydrocarbons and particulate matter; if the PCV system cannot handle the volume, these pollutants can vent directly to the atmosphere or into the intake tract, causing emissions failures and contributing to carbon buildup on throttle bodies and valves.
Diagnosis and Measurement
Identifying excessive oil blow by requires a systematic approach. A visual inspection for oil leaks around the valve cover and oil pan is the first step, though the presence of clean oil residue can sometimes mask the source. A more definitive method involves a differential pressure gauge connected to the oil filler cap or a specific test port. By measuring the pressure increase within the crankcase while the engine is running, a technician can quantify the severity of the blow by and determine if the issue lies with the rings, the ventilation system, or a combination of both.
