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.


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.

  • 1
    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
  • 3
    @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
  • 2
    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
  • I use a catch can to prevent hot oil/vapors from pooling in my nice cool insulated intake plenum, warming up the nice, cool air. I also, even if it makes near zero difference, would rather dump a little container of oil into my old oil bucket every once in a while then have to buy new o2 sensors or cats a little sooner. Jun 1 '16 at 23:29

What I see missing here is the fact that the PCV system does far more than just retain EPA compliance, the blow-by contains dozens of damaging compounds that must be removed as soon as they enter the crankcase by fresh air entering one bank, and the foul nasty vapors evacuated or "sucked" out the opposite bank while still in a gaseous state. If you do NOT constantly flush and evacuate these completely, then this water, sulfuric acid, un burnt fuel, abrasive soot and carbon particles are left to settle in the crankcase and cause sludge and greatly increased wear to the internal parts. The functions of a properly designed PCV system are many, and the emission portion is only one. Your engines would not last more than say 50-70 k miles like the old days (of course oil is far better formulated now) when these damaging compounds were left and only a draft tube was used.

  • Also missed: the tube from the air cleaner connects to the crankcase (generally through the valve cover). The air the PCV sucks out is replaced with filtered air drawn through that tube. Leaving the tube open means that the replacement air sucked in is unfiltered air, full of dust and whatever else. That dirty air puts dirt in the oil, and whatever isn't stuck in the oil also is fed into the intake by the PCV as if there wasn't an air filter to begin with.
    – dannysauer
    Oct 28 '19 at 19:50

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.

  • 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. 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. Jun 16 '11 at 13:31

I think everyone is missing the point of the catch can in the first place. The catch can sole purpose it to keep oil from going into your intake and being burn, which is BAD for the environment. You would put the catch can INLINE with the PVC return line to the Intake. When installed this way (the proper way), it is still recirculating the fumes, but lowering your emissions out of your exhaust due to you NOT burning oil anymore.

As far as what you do with the trapped oil, take it to AutoZone, O'reilly's, Pep Boys, Advanced Auto Parts... etc, they all accept used oil for proper disposal.

This simple method took my Gross Polluter, 230k+ mile, 1991 4Runner to passing smog out here in CA.


It is returned through the intake because the vacuum presure of the intake matches the crank pressure that needs to be released. You could. Run a mechanical or an electric vacuum pump to release the crank pressure, they say a mechanical pump is better, run of a belt it pumps more at high rpms and less at lower rpms. The right amount of vacuum at the right time. So I have read!. Apparently too little or too much vacuum is bad for the engine. May be not bad maybe just not as good as it could run

  • Not enough vacuum = blow your oil dipstick out of the tube or blow seals out. Too much vacuum = suck gaskets into the engine instead of blowing them out. There's definitely a right amount. :)
    – dannysauer
    Oct 28 '19 at 19:51

I think people are getting confused. There is something called a pcv/ccv which has a tiny hole and acts as a valve. Usually further away from the front of the vehicle.

Then, what I think OP is asking about, is basically a vent that goes into your air filter which is often misidentified as a pvc valve and ordered incorrectly because they look identical at a glance. It has a large hole and literally is just an elbow. They are not the same thing and If you accidentally swap them you will have problems.

  • 1
    There is no confusion. The PCV should not vent to atmosphere, as it contains blow-by vapor with combustion products and aerosolized oil. This would end up as polution, if it were not reintroduced to the intake stream in order to provide a change to re-oxidize and further catalyze the HC components before releasing to the atmosphere.
    – SteveRacer
    Dec 2 '17 at 6:17

Nobody got the point here. You are all talking about the environment. If you retain the pcv valve and connect a hose to the catch can, there wont be enough hole to trigger the pcv valve, it needs a vacuum, to pull the crankcase pressure. You want a "pcv delete". Drill the pcv valve, remove the spring and other valve mechanism inside so it will be just an air passage, completely hole, no valve, and connect hose to the catch can. Dont forget to plug the vacuum from the intake.


If you use an oil catch can designed to catch the oil and fuel vented from the engine in line of the the PCS valve hose and vent the other end into the intake you are still 100 percent legal. It will also help the engine because you are not coating the intake side of the engine and valves with oil and fuel deposits that over time rob you of power. I would keep doing some research on this if I were you. For new direct injection turbo vehicles I would recommend doing this to all of them. https://www.roadandtrack.com/car-culture/buying-maintenance/a27182434/how-oil-catch-can-works/

  • Hey Adam - please remember we have a Be Nice principle here. I'll edit your post to be a little less rude.
    – Rory Alsop
    Dec 29 '19 at 18:19

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