Engine Blow-By: Understanding the Basics

Understanding Engine Blow-By: What It Is and Why It Matters

If you’ve ever dealt with excessive oil consumption, reduced engine performance, or a mysterious oily residue in your intake system, you might have encountered a common yet often misunderstood issue—engine blow-by. It’s found in every internal combustion engine, even new ones. However, excessive blow-by can lead to serious performance and longevity concerns. In this article, I’ll break down exactly what engine it is, why it happens, which engines are more prone to it, and whether it’s something you should be worried about.

For more information on how much blow-by is too much, see this article.

What Is Engine Blow-By?

To put it simply, engine blow-by refers to the leakage of combustion gases past the piston rings and ring gap and into the crankcase. During the power stroke of an engine cycle, the air-fuel mixture ignites, producing high-pressure gases that force the piston downward. Ideally, the piston rings create a tight seal against the cylinder walls, ensuring all the energy is directed toward rotating the engine. However, some high-pressure gases inevitably escape past the rings, entering the crankcase. This is what we call engine blow-by.

Once inside the crankcase, these gases mix with oil vapors and cause several issues. Modern engines are designed with a Positive Crankcase Ventilation (PCV) system to recirculate these gases back into the intake manifold, allowing them to be burned off instead of accumulating in the engine. However, if the amount of these gasses is excessive, it can overwhelm the PCV system, leading to oil contamination, carbon buildup, and power loss.

Which Engines Produce Engine Blow-By?

Every internal combustion engine experiences some level of engine blow-by, but certain factors make some engines more susceptible to it than others.

• High-Mileage Engines—Over time, piston rings and cylinder walls wear down, creating larger gaps that allow more gases to escape. Due to this natural wear, engines with 100,000+ miles often see an increase in engine blow-by.

• Turbocharged Engines— Forced induction engines operate under higher pressures, which can accelerate piston ring wear. This means turbocharged engines are often more prone to engine blow-by, especially if proper maintenance isn’t followed.

• Diesel Engines— Diesel engines tend to have higher compression ratios than gasoline engines, leading to increased blow-by. Additionally, soot and carbon buildup can exacerbate ring wear, making blow-by more prevalent in older diesel engines.

• Engines With Poor Maintenance— Sludge buildup, infrequent oil changes, and neglected PCV system maintenance can all contribute to excessive engine blow-by. If the PCV system is clogged, blow-by gases have nowhere to go, leading to pressure buildup in the crankcase.

Is It Bad?

In small amounts, it’s normal. However, excessive amounts can lead to a variety of problems, including:

• Oil Contamination— When the gases mix with engine oil, they introduce unburned fuel, carbon deposits, and moisture, leading to sludge formation and reduced lubrication effectiveness.

• Carbon Buildup in the Intake System— If the PCV system sends excessive amounts of oil-laden blow-by back into the intake manifold, carbon deposits can form on intake valves, throttle bodies, and turbocharger components.

• Increased Crankcase Pressure— Excessive engine blow-by can create higher-than-normal pressure in the crankcase, potentially leading to oil leaks and seal failures.

• Reduced Engine Efficiency— When too much combustion gas escapes past the rings, it results in a loss of power and lower fuel efficiency.

How to Reduce Engine Blow-By

• Use High-Quality Engine Oil— Synthetic oils with good detergent additives help keep piston rings clean and minimize blow-by-causing deposits and wear.

• Change Your Oil On Time By Following the Correct Oil Change Schedule— All carmakers list a NORMAL and SEVERE service interval for oil changes. If you drive short trips in stop and go traffic, have long idle periods, haul heavy loads, live in mountanous areas, or drive in dusty conditions, you must follow the severe service oil change schedule to reduce engine wear.

If you’re experiencing excessive engine blow-by, there are steps you can take to mitigate its effects:

The Take Aways From This Article

A certain amount of blow-by is normal. You can expect more blow-by in diesel engines and gas engines equipped with a turbocharger.

Using the wrong oil or following the wrong oil change interval can wear the piston rings, causing excessive blow-by.

©, 2025 Rick Muscoplat