I have to vent about PCV valves

I will be back with more of the exciting alternator swap later. I haven't finished it yet. But for now, a little diversion based on a question I had from the last post.

One of my readers asked in an email about the function of the odd, nubbin-looking thing on the left, that seems to be leaking oil in this photo:

If you look closely, in the silver and red ribbed cover, there is a protruberance. What this is, is the end of a PCV valve that the previous owner thoughtfully plugged with a vinyl protective cap. Unfortunately, this renders it useless, and crankcase pressure is forcing oil and air out from around it. I will try to give you a simplified lesson in crankcase venting and emission controls relating to them.

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When a combustion engine works, some combustion (power) pressure leaks into the bottom of the engine (the crankcase) where the crank spins around, the pistons go up and down, and all the oil sits, waiting to be pumped around to lubricate everything. This "blowby" is a normal, unavoidable condition, but the air pressure buildup must be relieved, or it will cause the oil gaskets to get pushed out, and you will have huge oil leaks.

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On early cars, up until the late 50's, the venting was done with a tube that came out of the motor, and hung down under the car, and was open to the atmosphere. The air passing beneath the car in motion would help suck the pressure out, and for this reason this is called a "road draft tube". This also left oil mist all over the underside of the car, good for inhibiting rust, but bad for the environment.

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By the 60's, the PCV (Positive Crankcase Ventilation) valve was in use. This is a one way valve that came from the engine crankcase and was plumbed back into the intake, where the pressurized air would be burned up, along with the incoming fuel and air. The name comes from the idea that instead of just letting the pressure force it's way out passively, the PCV system would apply vacuum to positively pull the air/oil vapor out. This was better than just venting it out, but it does have a slight detrimental effect in that it can lead to increased deposits in the combustion chamber and on the valves. We can not make one change to the exclusion of others.

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On modern cars, I believe this pressurized oil mist is probably run into a charcoal cannister first to filter it, then ingested into the intake, minimizing intake contamination.

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The bottom line is that your engine makes internal pressure, and you have to let it out somehow. The owner of my car probably didn't know that the proper thing to do is to run a tube from the PCV valve into the bottom of the air cleaner, and instead just capped it off. We'll tidy this, and more, up later. Now I have to get back to that alternator install.

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If you have any questions, please ask. I will try to explain the basics in a simple fashion.