There's a term called blow-by.

  • What is blow-by?
  • What causes it to occur?
  • How does it affect your engine?
  • How do you cure it?

2 Answers 2


What is blow-by?

Blow-by occurs when the explosion that occurs in your engine's combustion chamber causes fuel, air and moisture to be forced past the rings into the crankcase. Your engine's rings must maintain an excellent fit in order to contain the pressure.

What causes it to occur?

As rings and cylinder liners wear away they are less capable of maintaining this seal. Consequently as a car ages the amount of blow-by that occurs can increase. Soot and deposits left over from incomplete combustion that collect on the rings can also inhibit their seal worsening blow-by.

How does it affect your engine?

Blow-by inhibits performance because it results in a loss of compression. When the expanding gases slip past the rings they cannot as effectively push the piston down and make the vehicle go. As a result the car will have less horsepower. This also results in a loss of fuel economy. When the fuel, air and moisture slip into the crankcase they contaminate and dilute the oil in the crankcase.

Among the many gasses in your compression chamber are unburned fuel, moisture, sulfur dioxide and soot. Once these gasses slip into your crankcase they can dilute into your engine causing great damage.

How do you cure it?

Frequent changing of the oil (which removes the carbon solids that erode the metal), adding detergents to the fuel and oil (that dissolve the solids into liquid form), using high quality oil and fuel, and ensuring that the combustion occurs correctly (which prevents the creation of hydrocarbon combustion by-product solids that wear the metal parts).

Generally once you have the problem, it's too late. You'd be looking at cleaning/replacing piston rings and cleaning/resurfacing the cylinder walls.


  • Love BobIsTheOilGuy.com. A wealth of great info about oil.. As for blow-by, it sounds like this is a death spiral. The engine in worn which causes blow-by, which dilutes the oil with the named contaminants, which causes damage, which causes more blow-by. And around in a loop till your engine is just too tired to move much at all. So a leak-down test is probably the best test for ring damage? Is there something one could do to test specifically for blow by? Maybe a chemical analysis of used oil?
    – cdunn
    Apr 11, 2017 at 19:37
  • It should be noted that a small amount of blow-by is normal for any engine; the rings cannot form a perfect seal, and this is what the PCV valve is for. Apr 11, 2017 at 19:48

Blow-by usually refers to combustion gases getting past the piston rings. Caused by excessive wear, soft rings, excessive fuel "washing" the oil off the bore or cylinder liner. As to how does it affect your engine - lack of power, oil leaks,excessive fuel and oil consumption, simply time for a re-build or replacement.

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