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After working on my car, I didn't bother to reattached the line from the pcv valve into the air intake before testing the motor. It ran normally. This made me question whether I need to even return it into the air intake.

Basically, can I run the hose from the pcv valve into a can or some filtered holder to collect oil, filter fumes or anything else coming out of the case? I would then cap the opening on the intake.

If doing the above is safe, would it actually help my engine run better? My logic as to why it might help, is it removes any oil that may otherwise go through the motor.

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2 Answers 2

up vote 5 down vote accepted

First: what you're suggesting is illegal if you live in a state where it's illegal to remove an emissions device. At a minimum, you're making your car non-street legal.

That aside, you're creating pollution for no good reason. There's a zero percent chance that your catch can will be more effective than a modern catalytic converter, especially when considered over the life of the vehicle. For example, how are you going to dispose of the contents of the can? That's a problem that a PCV-equipped car does not have.

So, to summarize:

Safe? Probably.

Run better? No.

More pollution? Yes.

Legal? Quite likely no.

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I think the admonishments on legality are a tad severe, but your answer is quite accurate. –  qes Jun 15 '11 at 23:02
    
First: It would be completely legal where I live to remove it. Second: You have answered my question. It is entirely for environmental reasons and isn't avoidable for street cars. Third: I am guessing the environment and legality are matters you take very seriously. To that, I say. Take it easy, I am not a criminal and in essence all I asked was, "Why do cars vent dirty air into the air intake? Can I use ... to avoid it?" My secret goal isn't to destroy the Ozone layer. –  Wulfhart Jun 15 '11 at 23:46
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@Wulfhart, legality in the US is a little complicated. You can easily get yourself into a situation where your modified car is legal within X square miles - don't cross the state line, though, or you'll have a problem. WRT to the environment, I look at it from the other direction: fiddling with the PCV line only creates pollution so why bother? I'm always strongly motivated by my desire to never be "that guy." –  Bob Cross Jun 16 '11 at 0:25
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I totally understand. I don't plan on being "that guy" either. I was legitimately curious as to why it had to return into the air intake. :-) –  Wulfhart Jun 16 '11 at 5:27
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The PCV system reduces pollution by collecting "blow-by" (unburned gasoline and oil vapor) that would otherwise be vented into the air we breathe. The PCV system routes blow-by back to the intake system so it can be burned properly. PCV is designed to handle these by-products correctly; it is not harmful to the engine.

Placing a closed container on the end of the PCV hose will not work. The engine constantly generates blow-by as it runs. Closing off the end of the PCV hose will raise the pressure inside the engine until the PCV hose (or one of its connections) bursts, your collecting container ruptures, the engine oil dipstick pops out, or one of your oil seals leaks or blows out.

Moreover, leaving the open hole on the intake side will throw your fuel/air mixture off slightly if the hole is downstream of the air mass meter. At a minimum, you're letting unfiltered, dirty air into the engine, which will cause it to wear out faster.

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You have it backwards from what I meant. The pcv would go into a filtering container trapped oil and vented a "theoretically" clean air. The air intake would be closed off. I am inferring from your response that it is purely environmental reasons why the air from the crankcase is not just released. Am I right in understanding that? –  Wulfhart Jun 15 '11 at 22:47
    
My original thought was that the air coming from the crankcase would throw the air/fuel ratio off as well as introduce oil into the airflow. Hence why some people use oil catch cans between the pcv and the air intake. –  Wulfhart Jun 15 '11 at 22:49
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@Wulfhart Thanks for the clarification. I think PCV is primarily, if not solely, an environmental measure. There may be some fuel efficiency gained by burning blow-by fuel vapor rather than venting it, but I suspect that is negligible. –  William Cline Jun 16 '11 at 13:23
    
@Wulfhart I would expect the automakers to design the engine to correctly handle the extra air coming from PCV. I don't think you're going to improve on their design. Likewise for keeping oil out of the intake. Remember that small amounts of oil leaks past the piston rings into the combustion chamber anyway. And for what it's worth, my 1992 Volvo's PCV system came from the factory with an oil reclamation mechanism built in. –  William Cline Jun 16 '11 at 13:31
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