What Is Engine Blow-by? What Does it Mean?

What Is Engine Blow-by?

Engine blow-by is when there is a leakage of air-fuel mixture or of combustion gases between a piston and the cylinder wall into the crankcase of an automobile.

Some signs of engine blow-by could be loud or sputtering noises coming from the engine, which could also be accompanied by clouds of exhaust or vented fumes. You’ve probably seen or heard it on the road as you’re driving or walking down the sidewalk and see a car or bus that has these signs. The same principle that is happening to the car or bus is essentially happening with your generator engine.

So how does this happen? It happens when the internal combustion that occurs in your engine’s combustion chamber causes contaminants such as air, fuel, and moisture to be forced past the piston rings within the crankcase. If your engine’s piston rings do not have a good fit, they cannot contain the pressure produced from the combustion, resulting in leakage of air-fuel mixture and contaminants.

It must be noted that engine blow-by does not solely occur in diesel engines, but can also occur within gas engines, as well.

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What Are Some of the Causes?

Over time, piston rings wear away, making them less capable of maintaining a firm seal. As the piston rings continue to wear away, the amount of blow-by can increase. Any soot and deposits that are left over from incomplete combustion will collect on the rings, interfere with their seal, and can further worsen engine blow-by.

Additional causes of blow-by:

  • Bad, broken, or improperly fit piston rings
  • Piston damage or defects
  • Gaps or space between the piston rings

How Does Blow-By Affect Engines?

Blow-by inhibits overall performance due to a loss of compression.

Blow-by coats parts of the intake with oil and fuel. After wear and degradation of piston rings and cylinder walls, more air, fuel, and moisture can slip into the crankcase.

When gases expand and slip past the piston rings, they cannot effectively push the piston down to make the engine produce sufficient output during the power stroke of the engine, resulting in a drop in horsepower.

Additionally, the engines will produce a much louder noise and can exhaust fumes that are forced out of the engine vents.

Blow-by will also result in increased fuel consumption. If contaminants such as air, fuel, and moisture slip into the crankcase, they can contaminate the oil and cause dilution. If there are any remnants of gases, unburned fuel, moisture, sulfur dioxide, and soot particulates left behind in your engine’s compression chamber. If these contaminants slip into your crankcase, they can seep into the engine and cause extensive damage.

Simply stated, engine blow-by will decrease the efficiency of your engine overall, result in costly and time-intensive repairs, or cause irreparable damage.

Preventative Maintenance

Proper maintenance of your engine is key to help prevent engine blow-by. Frequently changing the engine’s oil will help remove any solid carbon buildup, which are known to erode metal. Adding treatments and detergents to the fuel and oil will help dissolve solids and buildup into liquid form. Using high quality oils and fuels will also keep the engine running longer and will ensure that combustion occurs correctly. This will prevent the creation of solid byproducts like hydrocarbon combustion solids, which wear away metal. Regularly collecting and sending oil samples to have the oil checked for any foreign particulates and residue can also help inform you if there is any degradation occurring within the engine.

What If My Engine Has Blow-By?

Usually if your engine is experiencing problems of blow-by, it may be too late. The next step would be to clean and/or replace piston rings and cleaning or resurfacing of the engine block cylinder walls. Alternatively, it may be a prompt to replace the engine or genset altogether.

If you think your generator engine might be experiencing blow-by or if you’re looking for a new, used, replacement, or rental generator, please reach out to us at Woodstock Power Company for any questions or inquiries!

The Woodstock Power Company Experience

We have industry experts that focus on commercial power generator sets, with in-depth industry knowledge to help you choose the right generator that fits your needs.

Our experts are ready to assist you from answering any questions you may have about power generator sets to helping you find the best selection in our inventory based on:

  • Peak & Average Power Requirements
  • Fuel Preference (Natural Gas or Diesel)
  • Portability & Stationary Power
  • Prime & Standby Generator Requirements
  • Available Space & Exhaust Restrictions

Our experts are also able to help coach you on prime, continuous, and standby power generators, as well as finding the best surplus, new, or used generator that best fits your required application.

We carry only the most top rated new, used, and surplus power generator sets for sale, providing you with a great value generator that will fit your budget.

Our generators have been thoroughly inspected, serviced, and verified, which ensures that you will be buying a quality generator that you can depend on. If a generator is not up to par with industry standards, we make all necessary repairs or modifications, and fully test every generator before we sell. This ensures a full turnkey generator that’s ready to fire up and go!

Due to our wide selection of generator sets, we’re confident that we can find the model that best suits your operational needs.

We also buy used generators in good quality, if you have already upgraded and you’re looking to sell your older model.

Feel free to contact us with any questions, concerns, or inquiries to learn more about the Woodstock Power Company experience and the quality level of products & services we provide.

We can be reached at 610-658-3242 or you can send us an email at

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