The PCV Valve under the hood of most cars gives us all a chance for a breath of fresh air. What is it, what does it do and what are the symptoms of a bad one??

PCV - what does it stand for? Positive Crankcase Ventilation. Let's pick that apart a word at a time and see just what this little piece of hardware does. Let's start with the word Crankcase. The crankcase of an internal combustion engine is that portion of the engine that contains the crankshaft (no duh), the connecting rods, the under side of the pistons. It is also connected via passages in the casting of the engine block to the upper engine, the valve train, the camshaft and the timing gears and chain. The most critical part of the above components regarding the PCV is the pistons. Fuel and air enter the combustion chamber on the top side of the piston where the fuel burns and forces the piston down. That motion is converted into rotary motion of the crankshaft via the connecting rods and the crankshaft. That is what drives your car.

I mentioned that the combustion takes place in the combustion chamber. Ideally all the combustion gases which are in the combustion chamber stay there until they are exhausted via the tail pipe, however as engines get older (and even in brand new engines) some of the combustion gasses get past the sealing hardware on the pistons, the piston rings. As the engines age, the clearances between the rings and the cylinder walls increases and more of the combustion gasses escape into the crankcase. These combustion gasses contain acids and other harmful by-products of combustion which used to escape from the crankcase via a breather tube which exited into the environment. All that garbage going into the atmosphere makes smog and other breathing problems in our environment. In addition, the moisture in those gasses also caused sludge to build up in the engine, so the PCV system also helps reduce the sludge build up. So the engineers decided to capture all that stuff from the crankcase and ventilate it in a way that it becomes less harmful. Notice I said less harmful, not pristine. Notice also that we have covered the second word, Ventilation.

What's so positive about Positive Crankcase Ventilation?

It is called positive since there is a positive force that removes the combustion vapors from the crankcase. That force is vacuum - just like the vacuum that sucks the dirt out of your rugs and furniture only this vacuum is derived from the engine, not from a Hoover. Vacuum derived from the engine is a rather sensitive resource, that is you don't want to steal any vacuum from the intake manifold when the engine is idling. However, when the engine is running at highway speed the engine can afford to allow a "vacuum leak" to occur via the PCV Valve. It is a metered flow of air, controlled by a needle valve inside the PCV Valve. When the engine is at idle and vacuum is high the force of the vacuum overcomes the force of a spring inside the PCV Valve and it is closed down to allow a flow of about three cubic feet per minute. When the throttle is opened and the intake manifold vacuum is reduced (closer to atmospheric) then the spring in the PCV Valve forces the valve to open allowing up to six cubic feet per minute of flow to occur.

So, what goes wrong? First, the spring can become weak or break and the valve will not open at all. This results in no removal of engine vapors and a build up of pressure in the crankcase. The air will actually flow backwards into the air cleaner. One symptom of this is oil in the air filter housing at the entrance of the PCV tube. This can also occur if the Blow-By is too high. Second, the valve can get gummy and stick in the open position allowing too much flow at idle conditions. This results in a rough idle or a stalling condition. In either case, just whip out the Visa card and spring for a new one! They only a few bucks.

A quick check to see if it is faulty is to shake it rather violently and see if the ball inside rattles - if it does then it is probably OK. Then with the engine running, put your finger on the end of the valve. You should feel a strong vacuum. If you don't then the valve is clogged and/or the tube is leaking. Fix or replace as necessary.

Back to Brother Bob's Home Page

Copyright © 1996 by Bob Hewitt - All rights reserved