How Much Blowby Is Normal? Expert Insights for Engine Health

Blowby is a common issue experienced in engines and goes a long way in influencing their performance.

It’s essential for vehicle owners to understand how much blowby is considered normal to properly maintain their engine.

In this article, we’ll delve into the topic and help you grasp the fundamental aspects of engine blowby.

The amount of blowby that is deemed normal varies depending on factors like engine type and vehicle age.

For instance, an engine with low miles will typically have less blowby than one with higher miles. Similarly, some engine designs might be more prone to experiencing blowby than others.

Identifying the signs of excessive blowby is crucial for timely maintenance and optimizing your engine’s performance. Once you know the causes and symptoms, you can address any issues and ensure your engine functions in peak condition.

How Much Blowby Is Normal


Understanding Blowby in Engines

Combustion Process

The combustion process in an internal combustion engine involves the burning of fuel and air mixture in the combustion chamber. This creates high-pressure combustion gases, which push the piston down, generating power.

Engine Types: Gas and Diesel

There are two main types of internal combustion engines: gas and diesel. Gas engines rely on a spark plug to ignite the fuel-air mixture, while diesel engines use compression. Both gas and diesel engines experience blowby to some extent.

Blowby occurs when some combustion gases slip past the piston rings and enter the crankcase, causing pressure build-up. Minor levels of blowby are considered normal, as piston rings are not completely watertight even in new engines.

What Are The Key Causes of Blowby?

Worn and Damaged Piston Rings

Worn-out piston rings are a common cause of blow-by. Over time, these rings wear down and lose their sealing capabilities. This allows combustion gases to pass by the rings and into the crankcase, leading to blow-by.

Damaged piston rings can also contribute to blow-by. Excessive heat and pressure can cause the rings to break or lose their shape.

Poor lubrication or improper installation may lead to premature ring damage. Regular maintenance and monitoring of your engine can help prevent these issues.

How Much Blowby Is Normal


Cylinder Wall Issues

Cylinder wall damage is another factor in blow-by. This damage can be caused by excessive friction, resulting from insufficient lubrication or debris within the engine.

Damaged cylinder walls may have scratches, grooves, or other surface imperfections that prevent the piston rings from sealing properly.

Preventing and addressing cylinder wall issues is crucial for minimizing blow-by. 4 Regular oil changes and using high-quality lubricants can help reduce the risk of damage. If cylinder wall damage is suspected, a professional inspection is recommended.

How to Measure Blowby?

Here are two ways to measure blowby:

Leak-Down Test

A leak-down test is a popular method to measure blowby. It involves pressurizing the combustion chamber and observing how long it takes for the pressure to drop.

The test helps you determine the compression loss and identify excessive blowby in the engine. You can also use a compression tester to assess cylinder pressure and check for potential issues.

Cubic Feet per Minute

Another way to quantify blowby is by measuring Cubic Feet per Minute (CFM). This metric will help you understand the amount of blowby generated relative to the engine speed and horsepower.

For instance, in a street performance engine that produces 500 horsepower, you may expect about 10 CFM of blowby with conventional piston rings and end gaps. Ensure having proper crankcase vent and crankcase ventilation to prevent excessive blowby.

To measure CFM, attach a manometer to the crankcase breather tube and collect data by analyzing the engine’s behavior at different speeds. Monitoring CFM is crucial to maintain the engine’s performance and avoid issues caused by excessive blowby.

How Much Blowby Is Normal


Effects of Blowby on Engine Performance

Loss of Efficiency and Power

Engine blow-by occurs when combustion gases leak past the piston rings into the crankcase, leading to a loss of efficiency and power.

This can reduce the engine’s overall horsepower, making it less responsive. Even a small amount of blowby can harm performance, so it’s crucial to address it promptly.

Increased Fuel and Oil Consumption

When an engine experiences a blow-by, it can lead to increased fuel and oil consumption. The escaping gases can cause the air-fuel mixture to become unbalanced, resulting in poor combustion and higher fuel usage.

Additionally, oil can become contaminated by the gases, leading to increased oil consumption.

Higher Emissions

Blow-by gases not only negatively affect the engine’s performance but also contribute to increased emissions. When gases escape the cylinders, they bypass the emissions control system and directly enter the atmosphere.

This can result in higher levels of harmful pollutants, such as hydrocarbons and carbon monoxide.

Signs of Excessive Blowby

Blowby is a common occurrence in engines, but too much can indicate a problem. In this section, we will discuss the signs of excessive blowby and how they relate to your engine’s health.

White Smoke from Exhaust

One sign of excessive blowby is white smoke coming from the exhaust. This happens when gases, including oil, bypass the piston rings and make their way into the intake, leading to a mixture of oil and fuel burning in the combustion chamber.

This results in white smoke being emitted from the exhaust pipe. If you notice this while driving, it’s essential to have your engine checked for possible issues.

Low Oil Pressure

Low oil pressure can also be an indication of excessive blowby. The gases being forced into the crankcase cause pressure build-up. This can lead to oil being forced out of the engine through seals and gaskets, reducing the overall oil pressure.

Regularly check your oil pressure gauge or warning light, and if you notice a drop in pressure, it’s time to investigate and address the problem.

Spark Plug Fouling

Finally, excessive blowby can cause spark plug fouling. As blowby gases containing oil enter the intake, they can affect the air and fuel mixture. This can lead to a buildup of deposits on the spark plugs, causing them to foul and leading to poor ignition.

Regularly inspect your spark plugs and replace them if you notice a buildup of deposits. This will help to maintain your engine’s efficiency and reduce the risk of excessive blowby.

How Much Blowby Is Normal


Preventing and Managing Blowby In Your Vehicle

Regular Engine Maintenance

One of the best ways to prevent blowby is to perform regular engine maintenance. This includes checking and replacing the air filter and fuel filter as needed.

Clogged filters can lead to incomplete combustion, ultimately producing soot and deposits that contribute to blowby.

Checking and cleaning the PCV (Positive Crankcase Ventilation) valve is another essential step for engine maintenance. A malfunctioning valve can allow contaminants and moisture to build up inside the engine, worsening wear and tear.

Keep an eye on your oil level too, as low or contaminated oil increases the likelihood of blowby.

Using Additives

Using certain additives in your engine can help minimize blowby. Additives are designed to remove and reduce sludge, soot, and deposits, which come from incomplete combustion and oil contamination.

By using these additives, you can improve the sealing between piston rings and cylinder walls, slowing down the rate of blowby.

Choosing The Proper Oil

Choosing the correct type of oil for your engine is crucial in managing blowby. Oil plays a significant role in sealing the gaps between the piston rings and the cylinder walls.

High-quality oil reduces the amount of wear and tear and minimizes the accumulation of contaminants, sludge, and moisture. Make sure to follow the manufacturer’s recommended oil specifications for your specific diesel engine.

What Can You Do To Prevent Excessive Blowby?

Repair or Replace Piston Rings and Pistons

Excessive blowby could stem from worn-out or improperly installed piston rings and pistons. As engine builders would advise, it’s crucial to assess and determine the condition of these components. In the case of diesel engines, the diesel particulates can accelerate wear and tear.

As soon as you identify any damage or excessive wear, consider replacing the piston rings and pistons. It’s essential to address these issues, as ignoring them could lead to decreased engine efficiency, loss of power, and an increase in crankcase blowby.

Addressing Cylinder Wall Damage

Cylinder wall damage may also contribute to excessive blowby. These walls sustain damage due to the combustion process or carbon buildup over time. An inspection by a qualified engine builder can help you diagnose the issue.

If there’s a cylinder wall damage, it may require reconditioning or resurfacing. This process often improves the sealing between the piston rings and cylinder walls, effectively reducing blowby.

Moreover, adding extra features like a forced-induction system or an intercooler could help minimize blowby. Make sure that the breather tube is clear as a clogged breather tube can exacerbate blowby issues.


Understanding the normal level of blowby and identifying the causes and symptoms of excessive blowby is crucial for maintaining engine health. Regular maintenance, proper lubrication, and timely repairs can help prevent and manage blowby.

This ensures optimal engine performance, efficiency, and longevity. A proactive stance can help you minimize the detrimental effects of blowby and enjoy a smoother and more reliable driving experience.

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Sean Mendez

Hi, I am Sean, a self-confessed petrolhead. I live in Boise, Idaho with a busy family of four and our energetic Labrador retriever. Thank you for visiting my website. You can find my email on the contact page.

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