A catch can that needs emptying often is not doing the same job as one that needs it rarely, and the difference is often not the can. Before deciding it is undersized, work out what is inside it. The volume alone says little, because the same vapour produces very different liquid volumes depending on temperature, journey length and water content.
The useful comparison is against the car's own history, not against someone else's build. If the level rises faster than it did before a change was made, the change is the suspect. If it has always been fast, the installation or the engine's condition is the suspect. Those are different investigations.
Drain the can into a clear jar and leave it to stand. A separation into layers tells you almost everything. A watery, milky lower layer with oil on top is emulsion: water vapour condensing in the can, normal in cold weather and on short journeys, and made worse by a can that runs cool. A single dark oil layer is oil carried as mist.
The smell and the feel separate them further. Emulsion smells of fuel and water and rinses off with water; oil does not. If the can collects mostly emulsion, its position and temperature matter more than its size, because a can mounted where it stays cold condenses water that a warmer location would have passed as vapour.
The volume of vapour reaching the can is set by how much gas gets past the pistons into the crankcase. Worn rings, a damaged bore, a failing head gasket or a blocked breather all increase it, and the can is simply where it shows up. An aluminum catch can of any size fills if the engine pushes a lot of vapour through it.
A quick check separates condition from installation. With the engine idling, pull the oil filler cap and feel the flow. A light, even breathing is normal. A strong pulsing flow, or visible smoke, points at blow-by and moves the diagnosis into the engine. If the flow is heavy hot and under light load, the can is reporting a real problem.
A can fitted on the wrong side of the breather, or connected so that it sees manifold vacuum instead of crankcase flow, will collect far more than intended because it is pulled on continuously rather than breathing. The same happens when the can is plumbed into a PCV line that was never meant to pass that volume.
Routing matters as well. A long run of small-bore hose from the cam cover to a can mounted low and cool gives vapour time and surface to condense on, so liquid arrives that would otherwise have stayed airborne and been burned. Short, direct runs at a sensible height reduce collection. If the installation uses the fittings supplied with the can, check that none has a restrictor built in.
Empty the can, note the dipstick level, drive a normal week, and empty it again into the same jar. Note the volume and look at the layers. Then check the breather and PCV routes by hand for kinks, collapses and wrong-direction check valves, and confirm the can's inlet comes from the crankcase.
Repeat the week with the can's drain and vent confirmed clear. If the rate is unchanged and the contents are mostly oil, the engine is the source. If the rate falls once the routing is corrected, the installation was the source. If the contents are mostly emulsion, the can is running cold and the fix is position.
The can is a measuring device as much as a separator. Its job is to catch what the engine produces, and a fast fill is data, not a fault. Change the can or its size and the rate of production does not change; only the interval between emptyings does. So the sequence is contents first, routing second, engine third.
Once the contents and the routing are understood, the remaining question is whether the engine produces normal vapour or too much. Answer that with the filler cap check and with the trend over a few weeks. If the rate is stable and the contents are a mix of water and light oil, the installation is working and the interval is what this car does.
