What Is An Engine Blow-By? Know 5 Signs & 6 Steps To Fix It!

Written By: Terrence Hines
Category: Engine

“What is an engine blow-by?” is a common question. Engine blow-by is a common vehicle issue where gases leak from the combustion chamber into the crankcase.

This can lead to many engine problems if not checked and fixed promptly. Now, let’s delve deeper into understanding and resolving this issue.

What Is An Engine Blow-By?

Engine blow-by is a common condition that can occur in internal combustion engines. It happens when the combustion gases escape past the piston rings and enter the crankcase.

In a healthy engine, the combustion process keeps these gases confined within the combustion chamber, where they are utilized to power your vehicle.

However, as the engine ages and undergoes wear and tear, the piston rings may lose effectiveness and fail to seal properly. This can result in the occurrence of engine blow-by.

The severity of the blow-by can serve as an indicator of the overall health of the engine. When blow-by becomes more pronounced, it often suggests more significant engine wear or damage.

Monitoring blow-by levels can help identify potential issues with the engine and prompt necessary maintenance or repairs.

You can promptly address blow-by to ensure your engine’s longevity and optimal performance.

5 Signs Of Engine Blow-By

Several key signs indicate the occurrence of engine blow-by in a vehicle.

Signs of engine trouble include excessive oil consumption, decreased power and performance, increased exhaust smoke, and oil leaks or residue around the engine.

It is important to address engine blow-by promptly to prevent further damage and maintain the optimal functioning of the vehicle.

1. Excessive Oil Consumption:

If you notice that your car is consuming oil faster than usual, even though there are no visible leaks, it is possible that this could be a result of a blow-by.

Blow-by occurs when the combustion gases produced during the engine’s operation can cause the oil to burn faster.

What Is An Engine Blow-By?

Various factors, such as worn piston rings or excessive cylinder pressure, can cause this phenomenon.

It is important to address this issue promptly to ensure optimal engine performance and prevent potential long-term damage.

2. Reduced Power And Performance:

Blow-by, when combustion gases leak past the piston rings and enter the crankcase, can substantially impact your engine’s efficiency.

This can result in reduced engine power, leading to a noticeable decline in your vehicle’s performance and overall power output.

To ensure that your engine functions optimally and to preserve the longevity of your vehicle, it is crucial to promptly address blow-by and take necessary actions to minimize its effects.

By doing so, you can maximize the efficiency of your engine and enjoy a smoother and more reliable driving experience.

3. Smoke From The Crankcase Breather Or Filler Cap:

It could indicate a blow-by if you notice smoke emanating from the crankcase breather or oil filler cap.

In this condition, combustion gases, including unburned fuel and exhaust, escape past the piston rings and enter the crankcase.

This gas escape can lead to heightened oil consumption and reduced engine performance.

It is crucial to address this issue promptly to prevent further damage and ensure the continued optimal operation of your engine.

Taking immediate action will help maintain the longevity and efficiency of your engine, keeping it in top-notch condition for years to come.

4. Blue Smoke From Exhaust:

Blow-by is the unwanted occurrence of combustion gases leaking into the crankcase, which can lead to the emission of blue smoke from the exhaust.

This visible sign indicates the presence of oil burning during the combustion process, which could be a symptom of underlying engine issues.

Blue Smoke From Exhaust

Promptly addressing blow-by is crucial to optimize engine functioning, prevent damage, and minimize environmental impact from harmful emissions released into the atmosphere.

Preventing and managing blow-by can preserve engine performance and longevity while fostering a cleaner and healthier environment.

5. Excessive Pressure In The Crankcase:

If there’s a significant increase in crankcase pressure, it may be due to a blow-by.

Blow-by occurs when combustion gases, such as unburned fuel and exhaust, leak past worn or damaged piston rings into the crankcase.

This can result in abnormal pressure buildup within the engine. To check for blow-by, you can perform a simple test.

Remove the oil cap while the engine runs, and pay attention to any air puffing out. If you feel a noticeable flow of air, it suggests a blow-by is present.

Addressing blow-by is crucial for optimal engine performance. It reduces oil consumption, prevents potential engine damage, and ensures a longer vehicle lifespan.

6 Steps To Fix Engine Blow-By

Engine blow-by is a prevalent engine issue when combustion gases escape beyond the piston rings and enter the crankcase.

If not addressed promptly, this phenomenon can potentially cause substantial damage to the engine components. To rectify engine blow-by, it is crucial to follow a systematic approach.

Here are some detailed steps to effectively resolve this problem and restore the optimal functioning of your engine:

1. Identify The Problem:

To confirm engine blow-by, watch for signs like excessive oil consumption, reduced performance, smoke from the crankcase breather or exhaust, and high crankcase pressure.

These indicators suggest that combustion gases escape past piston rings and enter the crankcase.

This can lead to increased oil consumption, decreased engine performance, visible smoke, and higher pressure within the crankcase.

Paying careful attention to these details will help you identify and address any blow-by issues promptly and effectively.

2. Carry Out A Cylinder Leak-Down Test:

A leak-down test is a highly valuable diagnostic tool to pinpoint the exact source of gas leakage in an engine.

What Is An Engine Blow-By? Let's Find

This thorough test involves pressurizing each cylinder with compressed air and meticulously inspecting every component for any signs of leaks.

By identifying and fixing these leaks, you can ensure optimal engine performance and efficiency and prevent potential damage or issues in the future.

This meticulous approach to engine maintenance and troubleshooting can significantly contribute to the longevity and reliability of the vehicle.

3. Perform A Compression Test:

This detailed test helps evaluate critical engine components like pistons, rings, and valve seats.

By meticulously measuring the compression levels, this test provides valuable insights into your engine’s overall health and performance.

It allows you to identify potential issues early on and proactively address them.

During the compression test, if the results are below the manufacturer’s thresholds, it could indicate an engine blow-by. This condition needs immediate attention and further diagnostics.

Promptly addressing it ensures optimal engine performance and longevity, giving you peace of mind and confidence in your vehicle.

4. Replace Worn-Out Piston Rings:

If the piston rings are worn out, it is important to replace them to ensure optimal engine performance.

This crucial maintenance task involves carefully removing the pistons, accessing the worn-out rings, and replacing them with new, durable ones that fit perfectly.

By addressing this issue promptly and with precision, you can greatly enhance the longevity and efficiency of your engine, ensuring smooth and reliable operation for miles to come.

This not only improves the overall performance of your vehicle but also contributes to fuel efficiency and reduces the risk of further damage to other engine components.

Don’t underestimate the significance of this maintenance task, as it plays a vital role in keeping your engine running at its best.

5. Service Or Replace The PCV Valve:

The Positive Crankcase Ventilation (PCV) valve, an essential component in your engine, plays a crucial role in controlling the level of harmful gases.

It effectively regulates the internal pressure of the crankcase, preventing excessive blow-by.

Excessive blow-by can lead to decreased engine performance and increased emissions, impacting the environment and your driving experience.

Service Or Replace The PCV Valve

To keep your engine healthy and efficient, regularly service or replace the PCV valve to manage unburned fuel and combustion by-products effectively.

Doing so can contribute to a cleaner and more efficient running engine.

6. Regular Engine Maintenance:

Regular engine maintenance prevents engine blow-by, where combustion gases escape past piston rings and into the crankcase.

To effectively combat this issue, it’s crucial to perform timely oil changes using high-quality oil specifically formulated for your engine type.

This ensures optimal lubrication, minimizes friction, and maximizes engine performance.

By diligently following these steps, you can effectively address engine blow-by, ensuring your engine runs smoothly, efficiently, and with extended longevity.

Tips To Prevent Engine Blow-By From Happening

To prevent engine blow-by, where combustion gases leak into the crankcase, reducing performance and increasing emissions, it’s crucial to understand this phenomenon.

Regular oil changes using high-quality oil appropriate for your engine are the first line of defense.

As engine oil ages, loses viscosity, or becomes contaminated, it can contribute to blow-by. So, selecting the right oil and changing it as recommended is essential.

Additionally, perform routine maintenance checks on the engine’s critical components like piston rings, cylinders, and the PCV valve.

If these parts are worn or damaged, they can fail to seal the combustion chamber properly, leading to a blow-by.

Preventing Engine Blow-By
Regular Oil Changes
Wise Oil Selection
Maintenance Checks
Identifying Issues Timely
Timely Repairs/Replacements
Emission and Performance

Regular inspections can help identify potential issues early and allow for timely repairs or replacements, thereby preventing blow-by.


Understanding what is an engine blow-by is vital for maintaining efficient and smooth engine performance.

This phenomenon, characterized by the leakage of combustion gases into the crankcase, can lead to reduced engine performance and increased emissions.

It is crucial to incorporate regular maintenance practices to prevent and address engine blow-by.

This includes timely oil changes with high-quality oil, routine inspections of engine components, and prompt resolution of any identified issues.

Adhering to these measures can effectively avoid and rectify engine blow-by.

Terrence Hines

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