Blow-by consists of compressed air, unburned fuel, and exhaust gases that force their way past the piston rings into the crankcase during the combustion cycle. While a small amount is normal and managed by the positive crankcase ventilation system, excessive blow-by indicates worn piston rings or scored cylinder walls. Severe cases lead to lost compression, increased oil consumption, and crankcase overpressurization.
Pull the oil fill cap while it's idling; if it puffs smoke like a freight train, you've got massive blow-by.
A quick field test for excessive blow-by is to place a piece of paper over the open oil filler tube while the engine idles; if it gets blown away forcefully, the rings are likely shot.
Related terms
PCV
This emissions control system routes blow-by gases from the crankcase back into the intake manifold.
Compression
The phase in an internal combustion engine where the piston squeezes the air-fuel mixture.
Bottom-end
The lower section of an engine block containing the crankshaft, connecting rods, and main bearings.
Piston slap
A knocking sound caused by a piston rocking sideways against the cylinder wall.