Crankcase Ventilation (PCV) Valve Buying Guide
Last updated: 24 April 2026
The crankcase ventilation valve (also known as the PCV valve or breather valve) controls the flow of blow-by gases from the engine crankcase back into the intake system. It prevents pressure build-up in the crankcase that would cause oil leaks, and routes hydrocarbon vapours through the engine for combustion rather than releasing them to atmosphere. A faulty valve causes rough idling, oil leaks from seals and gaskets, increased oil consumption, and lean mixture fault codes. It is an inexpensive service item that should be inspected at regular intervals. At CarParts247 we stock crankcase ventilation valves for all popular petrol and diesel engines.
Symptoms of a faulty crankcase ventilation valve
CCV valve failure typically causes either excessive crankcase pressure (valve stuck closed) or vacuum issues (valve stuck open).
- Rough or unstable idle — valve stuck open, allowing unmetered air into the intake
- Oil leaks from rocker cover gasket, crankshaft seals, or dipstick tube — crankcase over-pressure
- Oil in the air filter housing or intercooler hoses — oil being pushed through the breather system
- Lean mixture fault codes (P0171/P0174) — excess air from open valve
- Thick black deposits around the valve and breather hoses — sludge from long oil change intervals
Inspection and replacement
The CCV valve is straightforward to inspect and replace. The replacement interval varies — some are replaced at every major service, others are replaced on condition.
- Shake the valve — a rattling sound on petrol engines indicates the check valve is free; no rattle may mean it has stuck
- Inspect breather hoses for cracks, softening, or oil saturation
- Clean the valve housing and breather gallery with solvent if sludge is present
- Replace the valve and any associated O-rings as a unit for reliability
- Use fresh engine oil at the specified service interval to reduce sludge formation
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Oil leaks, high oil consumption, and rough idle — how the PCV valve prevents pressure build-up, clogging symptoms, and easy replacement steps.
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Frequently Asked Questions
What does the crankcase ventilation valve do?
It regulates the flow of blow-by gases (combustion gases that pass the piston rings) from the crankcase back into the intake manifold, preventing pressure build-up and recycling hydrocarbons through combustion.
What are the symptoms of a stuck PCV valve?
A stuck-open valve causes rough idle and lean mixture codes. A stuck-closed valve causes crankcase over-pressure, leading to oil leaks from gaskets and seals, and oil contamination of the air filter and intercooler.
How often should the crankcase ventilation valve be replaced?
Many manufacturers recommend replacement every 30,000-60,000 miles or at major service intervals. Some vehicles have longer-life units — check your service schedule.
Can I clean a crankcase ventilation valve?
Mild sludge deposits can be cleaned with carb cleaner or solvent. However, if the check valve element is worn or the diaphragm is damaged, cleaning does not restore correct function. Replacement is more reliable.
Why is there oil in my air filter?
A faulty crankcase ventilation valve allows excess oil mist from the crankcase into the intake system. Oil in the air filter, intercooler pipes, or throttle body is a common symptom. Replace the CCV valve and clean the affected components.
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