What Is Engine Blow-by?

Engine Blow-By – What Is It?

Engine blow-by is a by-product of all internal combustion engines. It refers to the leakage of combustion gases past the piston rings in an engine and into the crankcase instead of being expelled through the exhaust system. Excessive blowby will negatively impact oil health and typically indicates reduced engine performance.

While a small amount of blow-by is considered normal in any engine (no engine maintains a perfect piston ring seal all the time), excessive blow-by indicates piston and cylinder wear or more serious problems.

Today we’re going to take a look at this important topic, focusing on crankcase blow by in diesel engines, specifically off-highway diesel engines.

We’ll provide an all-encompassing rundown of engine blow by, looking at what causes it, the signs of the problem, and how to prevent it.
Ready to go?
Let’s dive in!

Terms to Know

Definitions are important! Let’s make sure that we understand a few key terms as we look at the topic of engine blow-by.

1. Crankcase – The housing that encloses the engine’s crankshaft and other moving parts. It also serves as a reservoir for engine oil.
2. Pistons – Engine components that move up and down within the cylinders, compressing fuel and air for combustion.
3. Breather Filter (Crankcase Ventilation Filter) – A component designed to separate oil from blow-by gases, allowing only gases to pass into the intake system or vent.
4. CCV (Crankcase Ventilation Valve) – A check valve that allows pressure to escape from the crankcase but prevents pulsing between crankcase and air intake.

How Diesel Engines Have Changed 

Engine blow-by can be a problem when it causes elevated crankcase pressure that triggers oil leaks and other performance issues. But engine emissions regulations have led engine manufacturers to modify engine design, and this includes updates to crankcase ventilation systems. 

One of the most significant recent advancements in diesel engine technology has been the use of crankcase breather filters to manage blow-by gases. In older designs, these gases were vented directly into the atmosphere through an external breather vent tube. However, due to increasingly stringent EPA emissions regulations, modern engines now recirculate these gases back into the intake system. 

Benefits of This Change: 

• Creates a slight vacuum in the crankcase, which reduces oil leak tendencies. 

• Lowers total engine emissions by burning excess combustible gases. 

Potential Drawbacks: 

• Increased risk of oil being drawn into the intake, requiring an effective oil-air separator. 

• Breather filters can become clogged with oil contaminants, leading to restricted gas flow and increased crankcase pressure. 

• Excess crankcase pressure can lead to oil leaks, turbo failure, and increased oil consumption as oil gets pushed past seals and throughout the crankcase ventilation system. 

• A clogged air filter can create excessive crankcase vacuum, further increasing oil consumption as the engine pulls oil through the breather filter. 

So how does this impact engine maintenance today?
Proper maintenance of the breather system and crankcase ventilation components is now ESSENTIAL to avoid the problems of excessive crankcase pressure! 

Causes of Engine Blow-By

Excessive engine blow-by is usually caused by one or more of the following:

• Worn Piston Rings – This allows combustion gases to leak past and enter the crankcase.
• Damaged Cylinder Walls – This damage prevents proper sealing of the combustion chamber.
• Worn Valve Seals – Worn seals cause excessive oil consumption and blow-by.

Signs of Engine Blow-By

What are the signs of excessive engine blow-by in a diesel engine? You may know them already, but they include the following:

1. Engine leaking oil – Increased crankcase pressure forces oil past gaskets and seals.
2. Excessive vapor from the crankcase vent tube – More smoke indicates worsening blow-by.
3. Hard starting – Poor compression due to worn rings or cylinder walls.
4. Reduced engine power – Loss of compression reduces overall engine performance.
5. Increased crankcase pressure – This can lead to oil leaks and component failures.

All of these signs can indicate other problems as well, but engine blow-by is a definite possibility. And in the newer Tier 3 and Tier 4 engines, checking your Crankcase Breather Filter should be a top priority.

Dangers of Engine Blow-By

As you probably already know, ignoring engine blow-by can lead to severe consequences, including the following:

1. Catastrophic engine failure – Excessive wear and pressure can lead to eventual engine breakdown.
2. Excessive wear – Continuing to operate the engine can result in more costly repairs.
3. Decreased performance and efficiency – Poor combustion results in higher fuel consumption and power loss.

How Long Will an Engine Last With Excessive Blow-By?

We often hear this question. And the answer largely depends on how soon you identify the problem.

The lifespan of an engine with excessive blow-by will also vary based on how quickly the problem worsens. Pulsing vapor or rapidly increasing vapor indicates a more serious problem, such as a broken piston ring or melted piston. Gradually increasing vapor may not be a significant issue, as it is likely routine wear.

But either way, if left unaddressed, blow-by can be an indication of premature engine failure. Early detection and maintenance are the key to taking care of your engine properly!

Don’t risk engine failure! If you suspect that your engine has a problem, get it checked out today by a certified professional such as HooverTec!

Preventing Engine Blow-By

So how do we prevent excessive engine blow-by and excessive crankcase pressure in the first place? As in a lot of things, proper maintenance is key. Here are a few things to keep on your checklist:

1. Maintain the crankcase breather filter – Regularly clean or replace this extremely important filter in order to prevent crankcase-pressure-induced turbo failure and oil leaks.
2. Perform preventative engine maintenance – Monitor the condition of the air filter and intake system. Replace cracked hoses and check clamps to prevent dirt entering the intake.
3. Use high-quality fluids – Premium oil and fuel can reduce internal wear and contamination.

More Resources on Engine Blow-By

Interested in reading more?
For additional information and technical guidance, check out the HooverTec blog for in-depth resources on diesel engine maintenance and troubleshooting.

Wrapping It Up

Engine blow-by is a natural occurrence, but excessive blow-by and crankcase pressure indicate serious engine problems.

By understanding the causes, recognizing the warning signs, and implementing preventative maintenance such as maintaining your crankcase breather filter and air intake system, you can extend the life of your diesel engine and prevent inefficient operation and engine failure.

If you suspect your engine is suffering from excessive blow-by, don’t wait—take action to diagnose and resolve the issue today!

Reach Out Today!

Need professional assistance with your diesel engine blow-by problem?

We’re here to help!

Contact HooverTec today for expert help with parts and service that will keep your diesel engine running at peak performance!

Contributed by Stennett Eberly Certified Copywriter