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The PCV valve is an important part of a car's emissions control system. The acronym stands for Positive Crankcase Ventilation. The valve takes unburned "blowby" gases — gases that have escaped the combustion chambers by slipping past the pistons into the crankcase — and funnels them back into the intake manifold, where they ultimately rejoin the air-fuel mixture and are reburned. The PCV valve essentially recycles these escaped gases, reducing air pollution and preventing moisture buildup inside the engine.
In addition to protecting the environment, the PCV valve is also an important part of preventative care for a car's engine. Because it prevents moisture buildup inside the engine, the build up of sludge in the oil is also reduced, which causes engine oil to last longer and protects the inner workings of the engine. A valve that needs replacement may cause a rough idle as a result of pressure buildup within the crankcase.
Because of its role in regulating a car's emissions, a PCV valve in need of replacement can negatively affect a car's inspection results. Many mechanics recommend that it be replaced at around 30,000 to 50,000 miles (48,280 to 80,467 km). Specific car manufacturers may have different guidelines, so car owners should be sure to check their owner's manual and/or shop manual. A PCV valve is usually inexpensive and relatively easy to replace, and it makes a significant difference in the car's performance, so it might be advisable to replace the valve with every regularly scheduled tune up.
The PCV valve is usually located on or near the intake manifold. The valve will have a large rubber hose leading to it; the hose fits around the PCV, so most likely people will not be able to see the valve without removing the hose. To remove the hose, the hose clamp should be unscrewed and the hose worked loose. Once the hose has been removed from the PCV, an open-ended wrench can be fitted to the hexagonal base, and the valve unscrewed from the manifold. Note that the dirtier the PCV valve, the less distinct the rattle from the interior valve will be when it is shaken.