Crankcase emissions means airborne substances emitted to the atmosphere from any portion of the engine crankcase ventilation or lubrication systems.Checked 2026-08-12
Positive crankcase ventilation (PCV)
The system that routes gas leaking past the piston rings back into the intake instead of venting it.
Also called: PCV valve · blow-by · crankcase ventilation
What it means
Definition
Combustion gas that slips past the piston rings collects in the crankcase, the enclosed space around the crankshaft that also holds the oil. Federal regulation names what comes off it: crankcase emissions means airborne substances emitted to the atmosphere from any portion of the engine crankcase ventilation or lubrication systems. A PCV system draws that gas back into the intake to be burned rather than released, with a valve regulating the flow.
Why a buyer cares
What it changes
It is a small part with symptoms out of proportion to its size — a stuck valve can push oil past seals or unsettle the idle, and it appears on maintenance schedules rather than repair histories.
In practice
Where you meet it
On maintenance schedules and invoices as a PCV valve, and on emissions inspection checklists confirming the system is intact.
Two things that look alike
Not the same as
The EVAP system handles fuel tank vapour, not crankcase gas. Different hoses, different valves.
Read next
Related terms
Where this comes from
Sources
The regulation defines crankcase emissions, not the PCV hardware or the word blow-by. A Maryland inspection regulation was dropped in review: its text describes a carburettor-era system.