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What Causes Blow-By in a Diesel Engine?

September 21, 2023

Diesel engines are renowned for their power and efficiency, but like any mechanical system, they can experience issues over time. When your engine is running smoothly, it can be easy to push off regular maintenance. The longer your engine goes without service, the higher the risk of engine failure. An early warning sign that your engine needs some TLC is when you experience a blow-by. Every type of engine is capable of engine blow-by, but high-performance engines and diesel engines will exhibit signs early on. Curious about why your diesel engine is experiencing blow-by? Let’s dive into the basics and diagnosis, and how to significantly reduce engine blow-by.

Why do Diesel Engines have Blow-By? Causes and Symptoms

Engine blow-by is a result of a mixture of air-fuel or combustion gases leaking between the engine’s pistons and the cylinder wall, escaping into the crankcase. This mixture typically consists of water vapour, carbon dioxide, unburned fuel, and air. Blow-by occurs when the pressure in the combustion chamber becomes too great to be contained, forcing these gases into the crankcase. It’s essential to understand that some degree of blow-by is normal, but excessive blow-by can signal underlying issues. To minimize risks to your diesel engine, it is important to periodically check the state of your piston ring seal and monitor any changes. 

Diesel engines, in particular, are more prone to blow-by compared to gasoline engines due to their higher compression ratios, which create greater pressure in the combustion chamber.

Common Causes of Diesel Engine Blow-By

  • Piston Rings: Piston rings are crucial for creating a seal between the piston and the cylinder wall. Over time, as these rings wear down, the gap between the crankcase and the cylinder wall can increase, leading to increased blow-by.
  • Worn Cylinder Walls: While piston rings bear the brunt of wear, the cylinder walls can also deteriorate. As they wear down, they create a larger gap between the wall and the piston, allowing combustion gases to escape.
  • Scratching and Scouring: The core of your engine is just as susceptible to damage as any other part. Damage can be caused by dirt and debris caught in the cylinder, carbon in the oil, and leaking fuel injectors. Once the cylinder walls, pistons, and rings are damaged, it can create pathways for combustion gases to leak.
  • High Carbon Levels: Considered the silent killer of diesel engines, carbon build-up in the piston ring grooves or engines without electronic fuel injectors can increase blow-by.
  • Manufacturing Defects: In some cases, engine manufacturing defects, particularly in piston-to-cylinder clearances, or poor quality control, can lead to blow-by issues.

Engine blow-by can arise from a variety of factors and can easily be mistaken for other symptoms. However, understanding the symptoms of engine blow-by can help you plan and take preventative measures to protect your diesel engine and its inner workings.

During What Stroke Does Blow-By Occur in a Diesel Engine?

 Most diesel engines have a 4-stroke combustion cycle, with piston strokes for intake, compression, power and exhaust. Engine blow-by occurs when the combustion chamber hits the maximum pressure during the compression and power strokes. This reaction can be impacted by the engine temperature and whether or not it is under a load. The hotter the engine, the more likely it is to experience engine blow-by. 

As a rule of thumb, the average engine blow-by is between 1.5 and 3 cfm (cubic feet per minute) when the engine is hot and cold respectively.

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What Happens to a Diesel Engine Experiencing Blow-By?

Are you trying to identify why your diesel engine is acting up? To help you rule out other factors,  knowing the signs of an engine blow-by can save you time and money and minimize the risk of a misdiagnosis. Is your diesel engine experiencing any of the common symptoms of blow-by?

  • White Smoke: A telltale sign of blow-by is white smoke billowing from the oil-fill tube or valve cover opening.
  • Rough Idling and Misfiring: Diesel engines experiencing blow-by may exhibit rough idling and misfiring, indicating a loss of compression in the combustion chamber. An engine misfire occurs when combustion gases escape early from the chamber.
  • Increased Oil Consumption: If your diesel engine is using more oil than usual, it could be a result of blow-by contaminating the crankcase oil. Monitor your oil levels to ensure your engine will not seize and the timing of your oil change intervals.
  • Excessive Exhaust Smoke: Blow-by can contribute to excessive exhaust smoke, which is often thick and discoloured.
  • Poor Engine Performance: Overall engine performance may suffer, with sluggish acceleration and reduced efficiency. If you are experiencing reduced engine power, it can be a result of lost compression.
  • Increased Fuel Consumption: Decreased engine efficiency as a result of engine blow-by can result in higher fuel consumption rates and increased expenses. 
  • Runaway engines: In extreme cases of engine blow-by, your diesel engine can be at risk of running away and speeding out of control. If you notice any of the following symptoms, booking a service appointment should be your top priority.

How Do You Stop Blow-By in a Diesel Engine?

While it is nearly impossible to fully stop blow-by from happening, there are preventative measures you can take to mitigate the risk and ensure your diesel engine is in top condition. 

  • Optimal Ring Seal: Ensure that the piston rings create a tight seal between the cylinder wall and the piston. Regular maintenance is critical for medium to heavy-duty diesel engines that operate at high pressures and temperatures.
  • Ventilation: Use vent tubes or oil separators to manage extreme crankcase pressure and separate oil from combustion gases. Keep an eye out for a stuck-open valve, such as black smoke, engine sludge, or surging.
  • High-Quality Engine Oil: Use high-quality engine oil and remove stiction from piston rings to maintain a tight seal.
  • Regular Maintenance: Adhere to a strict maintenance schedule, including oil changes, injector replacements, and cylinder inspections. This will ensure that the pistons are more frequently monitored for signs of carbon buildup or contamination.

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