Glossary
PCV valve
The PCV valve routes crankcase vapor — blow-by gas that gets past the piston rings — back into the intake so it is burned instead of vented into the air.
The PCV valve handles the oil side of a car's emissions. Some of the gases in the cylinder always escape past the piston rings into the crankcase — blow-by — and they carry oil vapor with them. Left alone, that vapor pressurizes the crankcase, pushes oil past seals, and eventually escapes to the atmosphere. Positive crankcase ventilation closes that off: the PCV valve meters the vapor from the crankcase back into the intake, where it is burned with the incoming charge instead of vented.
It is a one-way metered valve rather than a filter, and that is why a stuck valve causes trouble at both extremes. Stuck shut, crankcase pressure builds and finds the weakest seal to push oil out of; stuck open, too much air is drawn into the intake and the engine idles badly.
The boundary on this subtest is the three emissions parts. The PCV valve is the only one dealing with crankcase vapor, the EGR valve handles exhaust gas, and the charcoal canister holds fuel vapor from the tank.
Taught in
Related terms
- EGR valve — The EGR valve sends a small, metered amount of exhaust gas back into the intake, which cools the burn and holds down the nitrogen oxides an engine makes when it runs hot.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)