Glossary
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.
The EGR valve exists to lower the temperature of the burn. Nitrogen oxides form when combustion temperature and pressure run high, because the nitrogen in the air and the oxygen in the charge combine at the peak of the power stroke. The cure is to dilute the charge: exhaust gas recirculation sends a small, metered amount of already-burned gas back into the intake, where it takes up space without adding fuel. The flame front then travels through a cooler, less energetic mixture, peak temperature falls, and nitrogen oxide formation falls with it.
It is not on all the time. Recirculation happens at part throttle — cruise and light load — where combustion runs hottest; at idle and at wide-open throttle the valve stays closed, because there the engine needs a stable, full-strength charge.
A failed EGR valve is a drivability fault as much as an emissions one. Stuck open, the engine stumbles and stalls at idle; stuck shut, it pings under load and fails the nitrogen oxide part of an emissions test, and the check-engine light comes on. The boundary against the PCV valve is where the gas came from: recirculated exhaust here, crankcase vapor there.
Taught in
Related terms
- 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.
- charcoal canister — The charcoal canister traps fuel vapor from the tank on activated charcoal instead of letting it escape, and holds it until the engine can purge the canister and burn it.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)