All cars nowadays have a positive crankcase ventilation system, the PCV system for short. The job of the PCV system is to direct crankcase vapors and blow-by gases into the intake manifold. From there they go into the combustion chambers and are burned.

Before cars had PCV systems, crankcase vapors and blow-by gases were vented from the engine directly into the surrounding air. So the PCV system is an emission control. It prevents these "dirty" gases and vapors from entering the atmosphere. (Blow-by gases, by the way, are the small amount of gases that seep down between the piston and cylinder wall into the crankcase during combustion.)

The PCV system is not complicated, but if it isn't serviced regularly, it can become clogged or malfunction. When this happens it can force engine oil out of the dipstick tube, and out the crankcase-breather hose into the air cleaner, coating the air-cleaner filter with oil and decreasing engine performance.

The PCV system should be checked at every engine tune-up, and the PCV valve should be replaced, even though it still may be good.

If you're not sure about whether your PCV system is working properly here's how to check it.

With the engine not running, remove the PCV valve from the valve cover. To remove it, simply pull it out. It will still be attached to its hose after you've pulled it out.

Shake the valve back and forth. You should hear a clicking sound.

Start the engine and let it idle. Put your thumb over the opening at the end of the valve, covering the opening. You should feel vacuum pulling against your thumb.

Insert the PCV valve back into the valve cover.

Now remove the oil filler cap. Place a sheet of notebook paper over the oil fill hole. The paper should be sucked against the valve cover when the engine is running.

If one or all of these checks are negative, chances are the PCV valve is stuck, or the PCV hose that connects the PCV valve to the intake manifold is plugged -- or both the hose and valve may be plugged.

To find out, remove the valve from the end of the hose. If there's a clamp holding the valve to the hose, remove the clamp first, then pull the valve out of the hose.

Remove the hose from the intake manifold. Again, if a clamp holds it to its connection at the manifold, remove the clamp first. Then simply pull the hose off.

Straighten the hose with your hands and look through it. If it's clogged, replace it. Also replace the crankcase breather filter at this time, and replace the PCV valve. Check the crankcase breather filter hose -- if it's clogged, replace it.

Basically the PCV system consists of the crankcase breather filter, its hose, the PCV valve and its hose. If these four items are new, then the PCV system should work.

After replacing PCV components, perform the checking procedures (thumb over valve, paper over oil-fill hole, shaking valve) mentioned earlier, just to be sure it's working.

The crankcase breather filter on some cars is a round cannister located in the valve cover. In many cars, however, the filter is located inside the air-cleaner housing. When you remove the top of the air cleaner you can see it.

If the carburetor air filter is dirty or oil-saturated, it should also be replaced when replacing PCV components.

In general, though, if the PCV valve and crankcase ventialtion filter are replaced at every engine tuneup, and you tune the car up at the intervals recommended in your owner manual or service manual, you'll never have any problem with this important emission-control device