When an engine's running, not all the air from its compression and combustion gets blown out the exhaust. This "excess" air is under pressure, and it has to go somewhere otherwise the pressure will force oil leaks all over. The solution is to have a way to blow-off or recirculate the pressurized air. But there's a problem in that this excess air contains tiny oil droplets that it collects from the engine's innards. Modern cars have fancy mechanisms for separating the oil from the air and recirculating both. The BMW 2002 is not a modern car.
Barring one of these modern mechanisms, there are some easy solutions to this problem. You can poke a hole in the top of the engine (the valve cover), attach a hose, and point the hose at the ground. Air moves freely, and the oil drains onto the ground. This solution's simplicity is beautiful, but it's inelegant, and, frankly, who wants to be leaving oil spots everywhere they park? The 2002's stock solution is to run a hose from the valve cover to the carburetor. The excess air and oil gets sucked back into the engine and burned off. That's great, but who want's gunk getting sucked into their carb?
Or, instead of running the hose to the ground or into the carb, you can run it to a catch can. The can collects the oil, and it has holes or a filter to let the air move. You can buy some really nice, beautifully fabricated oil catch cans for $40, $50, $70, $90, $100 . . . $130. A hundred and thirty dollars for a can? Seriously? It's a CAN!
We can do better. Hell, I have empty cans all over the place. Let's get DIY.
The whole point of the breather hose is to let the engine breath. Pressurized air needs to blow out of the valve train, so my catch can has to have some holes in it for air to move.
The hose slopes upwards, so most of the oil will probably drain back into the valve cover anyway. Still, I think it's a better solution than just sending the breather hose down to the ground and leaving oily spots everywhere I park. And it seems better than just putting a filter onto the valve cover and leaving an oily residue everywhere. It's certainly better than paying $100 for a fancy-pants catch can.
But it's not a perfect solution. I'll have to pull the lid every once in a while and check to see if oil's building up in there. There's no filter on the little holes, so they might still blow oil vapors everywhere. There's no valve at the bottom to drain oil, so I'll have to suck it out with a turkey baster or somethi . . . oh, who cares?! I've got a coffee can hooked to my engine! And it's functional! And it cost me nothing! So, there.