The PCV (positive crankcase ventilation) valve routes blow-by gases, the combustion gases that slip past the piston rings, from the crankcase back into the intake to be burned. It keeps crankcase pressure under control and pulls moisture and fumes out of the oil. A clogged PCV valve builds pressure that pushes oil past seals and gaskets; one stuck open acts like a vacuum leak and causes a rough idle.
How the PCV system works
Fresh air enters the crankcase through a breather, mixes with blow-by gases and is drawn through the PCV valve into the intake manifold by engine vacuum. The valve meters that flow and closes against backfire or boost pressure.
Bad PCV valve symptoms
- Oil leaks from valve cover gaskets and seals, from excess crankcase pressure; see oil leaks.
- Oil consumption or blue smoke, when oil is pulled into the intake; see exhaust smoke colors.
- Rough idle, whistling or lean codes from a valve stuck open or a cracked hose; see vacuum leak.
- Sludge in the oil, because moisture is not being vented.
- Oil in the air filter housing from pressure pushing back through the breather.
How to check and replace a PCV valve
- Find the valve, usually in a valve cover or in a hose between the engine and the intake.
- Inspect the hoses and grommet for cracks or softness.
- On many engines you can pull the valve and shake it: a rattle suggests it is free, silence suggests it is stuck. Some newer designs are built into the valve cover and are checked differently.
- Replace the valve and any hardened hoses; it is usually an inexpensive part.
How often should a PCV valve be replaced?
There is no single interval. Many owner manuals list it as an inspection item at regular services, and engines that make short trips or burn some oil clog it sooner. Because it is inexpensive and a clogged valve can cause oil leaks, replacing it whenever symptoms appear, or during other engine work, is sensible.
PCV systems on boosted engines
With a supercharger or turbo, the intake can be under pressure instead of vacuum, so the PCV system must handle both conditions without pushing oil into the intake. Supercharger kits often include parts for this; the East Coast Supercharging NOVI kits we carry, for example, list a PCV oil separator among their parts. See supercharger kits by platform.
Excessive blow-by itself is a sign of worn rings; see piston rings and compression test.
More guides: engine burning oil · milky engine oil · valve cover gasket leak · rear main seal leak.
Trouble codes: P0171 System Too Lean Bank 1 · P0174 System Too Lean Bank 2 · P0507 Idle RPM Higher Than Expected · all OBD2 codes.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.