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Purpose of the PCV system

As an engine runs, a small amount of combustion gases pass the piston rings into the crankcase. Without a means of extraction these gases pressurise the crankcase, forcing oil past gaskets and seals. The PCV system draws these gases back out and burns them, keeping crankcase pressure negative relative to atmosphere. This protects seals, reduces emissions, and prevents oil degradation from acidic combustion by-products mixing with the oil.

On modern engines the PCV function is often integrated into the rocker cover or a separate oil separator module rather than being a simple inline valve. These integrated units perform the same function but are more complex to replace.

Signs of a faulty PCV valve

A stuck-closed PCV valve causes crankcase pressure to build, pushing oil out through the rear crankshaft seal, rocker cover gasket, or dipstick tube. You may notice an oil leak that begins shortly after the valve fails. A stuck-open valve allows too much unmetered air into the intake, leaning the fuel mixture and causing rough idle, hesitation, and potentially a check engine light.

Sludge and carbon deposits in the PCV circuit are common on vehicles that have not been serviced at the correct intervals or that have used low-quality oil. The valve should be inspected and cleaned at major services.

• Oil leaks from crankshaft seals or rocker cover
• Rough idle or hesitation
• Increased oil consumption
• Black sludge in the engine oil
• Engine management light with lean fuel trim codes

Replacement

PCV valves are generally inexpensive and straightforward to replace — they typically push into a grommet in the rocker cover or clip inline between two hoses. Check the hoses and breather pipes at the same time, as they harden and crack with age and heat cycling. Use the registration lookup to confirm the correct PCV valve for your engine code and model year.