Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
up vote 10 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.

share|improve this answer
I think the admonishments on legality are a tad severe, but your answer is quite accurate. – quentin-starin 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
@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
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. – MooseLucifer Jun 1 at 23:29

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.

share|improve this answer
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
@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

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.

share|improve this answer

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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.