Excessive blow by represents a critical yet often misunderstood phenomenon within combustion engines, where combustion gases bypass the piston rings and leak into the crankcase. This condition, if left unaddressed, signals a departure from optimal engine health and can lead to a cascade of performance and mechanical issues. Understanding the nuances of this gas leak path is essential for maintaining the longevity and efficiency of any internal combustion powerplant, whether it resides in a daily driver or a high-performance machine.
Decoding the Blow By Mechanism
To grasp the implications of excessive blow by, one must first understand its intended, minimal occurrence. During the power stroke, the immense pressure generated by ignited fuel forces a small amount of gas past the piston rings into the crankcase. This is a normal event, managed by the crankcase ventilation system, which filters out combustible gases before reintroducing the oil vapor back into the intake. The problem arises when the volume of this leak surpasses the system's design capacity, indicating a failure of the sealing elements.
The Primary Culprits: Rings and Cylinders
The piston rings and cylinder walls form the dynamic seal that contains combustion pressure. Excessive blow by is most commonly attributed to wear or damage within this critical interface. Over time, the rings lose their tension, become brittle, or break, while the cylinder bores develop micro-scoring or become excessively worn. This physical gap creates a direct highway for high-pressure gases, allowing them to escape past the piston and overwhelm the ventilation system.
Performance and Mechanical Consequences
When blow by exceeds acceptable levels, the engine’s performance begins to suffer immediately. The leaking gases rob the combustion chamber of pressure, directly translating to a loss of power, torque, and fuel efficiency. The engine may feel sluggish, hesitate under load, or fail to maintain desired RPMs as the pressure dynamics within the cylinder are disrupted.
Increased oil consumption due to combustion gases forcing past the rings and contaminating the oil.
Dilution of the lubricant, which reduces viscosity and film strength, accelerating wear on bearings and other critical components.
Increased pressure and heat within the crankcase, stressing seals and gaskets.
Potential failure of the turbocharger, where oil contamination leads to imbalance and bearing failure.
Diagnosis and Measurement
Identifying excessive blow by requires a systematic approach that moves beyond a simple visual check. A mechanic will typically perform a differential pressure test using a gauge connected to the dipstick tube. This provides a quantifiable reading of the pressure buildup within the crankcase, allowing for a definitive diagnosis. Furthermore, a thorough internal inspection via borescope can reveal the condition of the cylinder walls and the integrity of the piston rings, confirming the source of the leak.
Restoring the Seal
Addressing the root cause is the only effective solution to excessive blow by. For many engines, a comprehensive ring job is the necessary remedy. This involves removing the pistons, installing new, correctly tensioned rings, and honing the cylinder walls to create a perfect surface for the new components. While this represents a significant intervention, it restores the engine’s compression, efficiency, and reliability, effectively eliminating the unwanted gas leak and preserving the machine for years of service.
