Originally Posted by AzPacknHorns
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.
The causes of blow-by: wear, soot and deposits
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.
But the easiest way to think about it is that it is "worn out"