Piston Rings

How to Fix Blow By on a Diesel Engine: A Comprehensive Guide

How To Fix By Apr 21, 2023 No Comments

As a diesel engine owner, experiencing blow-by can be a frustrating and costly problem. Blow-by occurs when combustion gases escape past the piston rings and enter the crankcase, leading to decreased engine performance and potential damage if left unaddressed. However, with the right knowledge and tools, fixing blow-by on your diesel engine is possible. In this article, we will discuss the causes of blow-by, how to diagnose the issue, and various methods for fixing it.

What Causes Blow-By on a Diesel Engine?

Blow-by can be caused by a variety of factors, including:

  • Worn piston rings – Over time, piston rings can wear down and lose their ability to create a proper seal between the piston and cylinder wall. This results in combustion gases escaping past the rings and into the crankcase.
  • Cylinder glazing – This occurs when the cylinder walls become polished and smooth, reducing their ability to hold oil and create a proper seal with the piston rings.
  • Worn valve guides – Valve guides can wear down over time, causing the valves to not seat properly and allowing combustion gases to escape.
  • Cracked pistons or cylinder walls – In extreme cases, blow-by can be caused by a cracked piston or cylinder wall, allowing combustion gases to escape into the crankcase.

Piston RingsSource: bing.com

Diagnosing Blow-By on a Diesel Engine

Before attempting to fix blow-by on your diesel engine, it’s important to diagnose the issue to ensure that blow-by is indeed the problem. Here are some common signs of blow-by:

  • Excessive oil consumption – If your engine is burning through oil at a rapid rate, it may be a sign that combustion gases are entering the crankcase and diluting the oil.
  • Smoke from the exhaust – Blow-by can cause excessive smoke from the exhaust, including white or blue smoke.
  • Loss of power – Blow-by can cause a loss of power and reduced engine performance.
  • Increase in oil level – If the oil level in your engine increases without any oil leaks, it may be a sign that blow-by is occurring.

Once you’ve identified these signs, you can perform a compression test to confirm the presence of blow-by. A compression test measures the pressure in each cylinder and can help determine if the piston rings or cylinder walls are worn. To perform a compression test:

  1. Remove the glow plugs or injectors to relieve pressure in the cylinders.
  2. Connect a compression gauge to the first cylinder and crank the engine.
  3. Record the pressure reading and move on to the next cylinder.
  4. Compare the readings to the manufacturer’s specifications to determine if there are any significant differences between cylinders.

Compression TestSource: bing.com

Methods for Fixing Blow-By on a Diesel Engine

Once you’ve identified blow-by as the issue, there are several methods for fixing it:

Replacing Piston Rings

If worn piston rings are the cause of blow-by, replacing them is the most effective method for fixing the issue. Here’s how to replace piston rings:

  1. Remove the cylinder head and oil pan to access the pistons.
  2. Remove the piston from the cylinder and remove the old piston rings.
  3. Install new piston rings according to manufacturer specifications.
  4. Reinstall the piston and cylinder head.

Replacing Piston RingsSource: bing.com

Honing Cylinder Walls

If cylinder glazing is the issue, honing the cylinder walls can help restore the proper surface finish and improve the seal between the piston rings and cylinder walls. Here’s how to hone cylinder walls:

  1. Remove the cylinder head and oil pan to access the pistons.
  2. Use a honing tool to remove a small amount of material from the cylinder walls, creating a crosshatch pattern.
  3. Clean the cylinders and reinstall the piston rings and pistons.
  4. Reinstall the cylinder head and oil pan.

Honing Cylinder WallsSource: bing.com

Replacing Valve Guides

If worn valve guides are the issue, replacing them can help restore proper valve seating and reduce blow-by. Here’s how to replace valve guides:

  1. Remove the cylinder head and valves.
  2. Use a valve guide removal tool to remove the old valve guides.
  3. Install new valve guides according to manufacturer specifications.
  4. Reinstall the valves and cylinder head.

Replacing Valve GuidesSource: bing.com

Replacing Pistons or Cylinder Walls

If the blow-by is caused by a cracked piston or cylinder wall, replacing them may be necessary. This is a more involved process and may require professional assistance. Here’s a general guide:

  1. Remove the engine from the vehicle and disassemble it.
  2. Remove the pistons and cylinder walls.
  3. Inspect all components for damage and replace any that are damaged.
  4. Install new piston rings according to manufacturer specifications.
  5. Reassemble the engine and reinstall it in the vehicle.

Replacing Pistons Or Cylinder WallsSource: bing.com

Preventing Blow-By on a Diesel Engine

While blow-by can be caused by a variety of factors, there are steps you can take to prevent it from occurring:

  • Perform regular oil changes – Regular oil changes can help keep your engine running smoothly and prevent cylinder glazing.
  • Replace worn parts – Keeping your engine well-maintained and replacing worn parts as needed can prevent blow-by from occurring.
  • Use high-quality fuel and oil – Using high-quality fuel and oil can improve engine performance and prevent damage to engine components.


Experiencing blow-by on a diesel engine can be frustrating, but with the right knowledge and tools, it’s possible to fix the issue and prevent it from occurring in the future. By identifying the cause of blow-by and using the appropriate method for fixing it, you can keep your diesel engine running smoothly and efficiently for years to come.

Related video of How to Fix Blow By on a Diesel Engine: A Comprehensive Guide


No Comments

Leave a comment

Your email address will not be published. Required fields are marked *