Oil Catch Can Benefits Debated - Why?

Why do people debate if oil catch cans are beneficial or not within the PCV system. It seems like you can’t really debate it, if you remove oil/gas mixture from going into the intake manifold to get burned, creating carbon buildup on the intake valves. I know it doesn’t remove all of the oil/gas mixture from the air, and you’ll still get some going into the intake. But if you remove at least some of it and can, then why not? Why is it debated if there is any benefit or not in oil catch cans? It seems like a no brain modification that is cheap and easy to do, with no real reason not to do it.

Limited engine compartment space, potential fire hazard, and another maintenance item that would increase the “cost to own” number that some potential buyers use when selecting a car. Those are the only downsides I can think of. Besides reducing gunk in the throttle body, from a pollution aspect, your ideas seems like it may have some merits too. I expect you are over-estimating the throttle body gunk improvement though.

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I am not familiar enough with them to say, but I don’t recall that part being on any car I have owned.

Who are these people that are debating this ? Sounds like device that would need maintenance but probably not get as often as it should.

Not long ago there were catch “can” filters for the crankcase vent


whats the difference between ventilation valve and pcv?

Since I have two direct injection engines, I have read some about them. I felt it is more hype than anything else, like K&N cold air intake and such. If it was going to save these engines, then the manufacturers would spend the extra $20 and put one in them to reduce their warranty claims.

Are we talking breather caps like was on our 58 Chevy? A cap with some filter material in it connected to the tube pointing at the ground. Once in a while you’d have to burn the oil out of the cap so it could breath again.

Yes, and eliminating those breather tubes was the first emission control implemented. You don’t need a catch can unless the rings are worn and you are getting excessive blowby and then I cannot imagine any such device being effective without venting to the outside air, violating emission standards.

Catch cans collect the excess oil mist blowby in the crankcase evap recirculation system. They are sealed systems, not vents. If the engine is in good shape and doesn’t use much oil they are of limited benefit. Considering some car makers think 500 miles per quart oil consumption is OK, there is benefit to adding a catch can.

My non direct injection turbo Saab had one. It was more of a box than a can but did the same thing snd drained the oil back into the oil pan. My current direct injection turbo Audi would cost about $500 to buy a catch can system from waht I have found. Doesn’t use oil so no need yet. Plus I have run Seafoam through the intake and will continue.

Considering how expensive it is to clean the front sides of the valves if they get carboned up, seems it couldn’t hurt.

I’m watching one guy who is in engine remanufacturing business in Europe and he regularly posts “autopsies” for various older engines.
Interestingly enough, some VAG designs incorporate this “can” concept into their valve covers, where they have an elaborate labyrinth for the blowby gasses to pass and to settle as much oil as they can before dumping it to PCV.

I haven’t had my valve cover off, but that sounds promising!

Blow-by isn’t a new phenomenon. Apparently my old Saab had 6 revisions to the crankcase liquid recovery before they found a cure. I never had the problem until #6 was out. The issue was clogging the crankcase vent over time and pressurizing the entire engine internals when the turbo was under boost. Oil would then find its way out the weakest spot. On our car, that was the front crank seal. Blew so much oil out the front, the serpentine belt jumped to its death. I installed catch-box version #6 and all was well.

The concept is not new and there are multiple takes from the multiple manufacturers.
I recall working on one of my older Nissans and they had the thing with channels and some “sponge” material at the very end to get the final oil mist before PCV and that thing was restricted due to the age.
A few squirts of carb cleaner fluid fixed that on the spot with tons of gunk washed out, it fixed my oil seepage issue.
So, answering to OP: don’t touch the PCV system, the manufacturer knows better, if it does not work, fix what is broken :slight_smile:

Here’s a study showing the elimination of PCV fumes did not reduce intake valve deposits:

Combustion gasses can travel past the intake valve and into the intake manifold at the beginning of the intake stroke.
This is how many engine designs have eliminated the EGR valve; and it’s also a possible source of intake valve deposits.

Not sure what a “ventilation valve” is, but before PCV systems became commonplace car engines still needed the crankcase gasses removed. That was done with a downward oriented tube from the engine that was open at the other end. The open end terminated slightly below the engine so it was in the under-car air-stream. The air rushing by the open tube, via the Bernoulli effect, would draw any gasses out of the crankcase.

It’s obvious that the PCV being POSITIVE is confusing. If that valve were eliminated the vent to the breather would take care of eliminating crankcase blowby except when well worn engines are at idle for extended periods. It is absolutely false that the PCV system operates by drawing blowby from the engine through the PCV valve while drawing fresh air in through the vent from the air cleaner.

That was called a “road draft tube”.
They would leave an oily stain down the middle of road lanes.

I’m not sure what you mean. Are you saying the diagram of the vapor flow directions in the diagram from link below is incorrect?

And that’s a dream world @George_San_Jose1. Under hard throttle, even cruising at expressway speed, the manifold vacuum drops to the mid teens or less while the blowby increases substantially. The carbon and oil that accumulates in the plumbing shown in the drawing as ‘fresh air’ intake is indicative of crankcase vapors being expelled through there. Not even in the most optimistic scenarios would a PCV valve flow enough to result in the flow seen in that picture.

FWIW I found this

catch can ad.