PCV Valve Maintenance: The $5 Fix That Protects Your Engine and MPG

What the PCV Valve Does

The Positive Crankcase Ventilation (PCV) valve routes blow-by gases from the crankcase back through the intake manifold to be re-burned. Without it, crankcase pressure would build and force oil past seals and gaskets. The PCV also prevents these gases from venting to atmosphere (an EPA-regulated emission since the 1960s).

How PCV Failure Hurts Economy

A stuck-open PCV valve (the more common failure mode) allows too much crankcase gas into the intake. These gases are rich in unburned fuel and oil vapor, enriching the intake mixture beyond the ECU's compensation range. Fuel trims run rich, O2 sensors see rich conditions, and economy suffers — often by 5–10%.

A stuck-closed PCV valve allows crankcase pressure to build, pushing oil into the air filter housing and intake. Oil in the intake coats the MAF sensor, throttle body, and intake valves, causing all the problems described in those maintenance articles.

Diagnosis

Remove the oil filler cap with the engine running at idle. There should be a slight vacuum pulling at the opening (suction on your hand). Positive pressure (gases blowing out) indicates a stuck-closed PCV. Inspect the PCV valve: it should rattle when shaken. A silent PCV valve is stuck — replace it.

Replacement

PCV valves cost $5–$20 at any parts store. Replacement takes 5 minutes on most engines — pull the old one from the valve cover grommet, press in the new one. Include this in every major service interval.