Blow-by in Diesel Engines: Causes, Diagnosis, and Common Questions

Blow-by is a term familiar to many in the automotive industry, especially those dealing with diesel engines. This phenomenon, while common, can be a cause for concern among vehicle owners and mechanics alike. In this comprehensive guide, we’ll explore what blow-by is, how to check for it, its causes, and whether all engines experience it.

What is Engine Blow-by?

Blow-by occurs when combustion gases escape past the piston rings into the crankcase of an engine. In a diesel engine, this typically happens during the compression and power strokes of the piston. These gases, which consist of unburned fuel, air, and moisture, can lead to increased pressure in the crankcase and a host of engine issues.

In a perfectly sealed combustion chamber, all the gases would be forced out through the exhaust valve. However, no engine is perfectly sealed, and some level of blow-by is normal, especially in high-mileage engines. But excessive blow-by can be a symptom of underlying problems.


How to Check for Diesel Engine Blow-by

  1. Oil Filler Cap Test: One simple method to check for blow-by is to remove the oil filler cap while the engine is running. If you notice a significant amount of smoke or gas coming out, it might indicate excessive blow-by.
  2. Dipstick Test: Similar to the oil filler cap test, you can also remove the dipstick while the engine is running. Again, visible smoke or gas emissions can be a sign of blow-by.
  3. Crankcase Pressure Test: More accurately, you can measure the crankcase pressure using a manometer. Higher than normal readings could indicate a problem.
  4. Visual Inspection of Components: Look for signs of oil leaks, especially around seals and gaskets, as high crankcase pressure from blow-by can force oil out of these areas.

What Causes Engine Blow-by?

Several factors can contribute to excessive blow-by in diesel engines:

  1. Worn Piston Rings or Cylinders: The most common cause is wear and tear on piston rings or cylinder walls. This wear creates gaps for gases to escape.
  2. Clogged Crankcase Ventilation: If the crankcase ventilation system is clogged, it can cause pressure to build up, leading to blow-by.
  3. Engine Overheating: Excessive heat can cause components to expand and warp, leading to increased blow-by.
  4. High Mileage: Over time, engine components naturally wear down, which can increase the amount of blow-by.


Do All Engines Have Blow-by?

The short answer is yes, to some extent. All combustion engines, whether diesel or gasoline, experience some degree of blow-by. It’s a natural byproduct of the combustion process and the fact that no engine is perfectly sealed. However, the amount of blow-by can vary greatly depending on the age of the engine, its design, and how well it has been maintained. Diesel environmental emission standards should always be monitored.

In new or well-maintained engines, the blow-by is minimal and doesn’t usually cause any issues. However, as engines age and wear, the amount of blow-by can increase. This is why regular maintenance and monitoring are crucial, especially for diesel engines that are often subjected to more strenuous conditions than their gasoline counterparts.


Blow-by in diesel engines is an issue that shouldn’t be ignored. While a small amount of blow-by is normal and expected, excessive blow-by can indicate serious problems that need addressing. Regular checks for signs of blow-by should be part of your engine maintenance routine. If you do detect signs of excessive blow-by, it’s advisable to consult with a professional mechanic to diagnose and resolve the issue.

Understanding the causes and symptoms of blow-by can help you maintain your diesel engine more effectively, ensuring it runs smoothly and lasts longer. Remember, the key to a healthy engine is not just in addressing issues as they arise but in preventing them through regular, proactive maintenance.