How to replace a PCV valve:

By Roger Houghton

A positive crankcase ventilation (PCV) system was one of the earliest emission-control devices. It draws left-over combustion gases from the crankcase that have escaped past the piston rings and routes them back into the engine, where they are burned in the combustion chambers, instead of escaping into the atmosphere.

The PCV system is seldom listed as a maintenance item, but it can cause performance and emissions problems, such as irregular engine idling and sluggish acceleration. A blocked PCV valve can also result in increased oil consumption as it pressurises the inside of the crankcase and can force oil out of the seals meant to keep the oil on the inside.

A valve is at the heart off the PCV system and is usually mounted on the valve or camshaft cover. It is either screwed in or a push fit in a rubber grommet.

Although seldom on a maintenance schedule, it pays to check the PCV system when doing routine maintenance such as changing oil and filters.

Usually if you shake the valve and hear a metallic sound then it is OK, but if not, it may be clogged with oil sludge and must be cleaned with a solvent such as an engine cleaner, or replaced. The connecting pipes should be cleaned at the same time.

This is another component that service centres often tell unknowing car owners needs replacing, when it is not necessary.