Thought I'd put up some info even though it isn't on the cruze (yet) since some members are seeing oil in their charge pipes.
Basically what happens is there is a Positive Crankcase Ventilation (PCV) valve that allows vapors in the crank case (where your oil sits) to be vented back into the intake
The blowby vapors that end up in an engine's crankcase contain moisture as well as combustion byproducts and unburned fuel vapors. The crankcase is sealed to prevent the escape of these gases into the atmosphere, but the vapors must be removed to prevent oil contamination that leads to sludge formation. The positive crankcase ventilation (PCV) system siphons these vapors from the crankcase and routes them into the intake manifold so they can be reburned in the engine.
but what can happen is oil can also escape and get into the intake. Sometimes it will make it's way into the cylinders and cause blue smoke to come out of the tail pipe and gunk up the motor. Since oil has a low octane rating, it will cause knock/detonation which is the premature combustion of the compressed air/fuel mixture. sometimes this can be heard as an audible "ping" like there are pennies rattling inside the engine.
To prevent this, there is a device called an "Oil Catch Can" and it's purpose is to take in the air/oil coming from the PCV valve and capture the oil while letting the vapors continue into the intake for burning.
This is the catch can I bought for my SRT-4. It's a nicer one, but it's pretty much universal.
It has two ports, one for the line in from the PCV and another for the line to the throttle body/intake manifold.
it also has a release valve on the bottom for quick draining. Also on the side is a clear tube that gives a quick visual of how full that thing is.
The way it works is it has a divider wall on the inside and both sides are stuffed with steel wool. Vapor goes in and out while oil stays in.
High boost setups
In high boost setups (basically anything beyond stock settings), you get what's called pressurizing of the valve cover. This happens under high boost and is caused by the boost pressure going in the wrong direction up the PCV line and forcing its way past the PCV valve and into the valve cover. What happens then? Well there could be a few things like oil being forced past valve seals and down into the head(s) and back into the cylinders (read above) but the most common is that the pressure exits through the vent. Most vehicles have vents in the valve cover and surprisingly, a lot of them do not have shrouds to prevent oil from escaping. The vent serves a similar purpose as the PCV, allowing for pressure to escape and are usually piped directly into the intake again (for emissions reasons). In the V style motors like the V6 and V8, it's common for people to remove the PCV and put a breather which looks like a tiny K&N filter on top of one or both valve covers.
anyway, the pressure from the boost getting past the PCV causes oil to fly out of the vent or keep the crank case/valve cover under pressure (which is bad all around). To prevent this, a 1-way check valve is placed in between the throttle body/intake manifold and the catch can. While you can install a check valve without a catch can, it isn't recommended as your check valve will get oily and fail (usually from getting clogged by oil). What the check valve does is allows air to flow only in one direction and not the other. so when installed correctly, the pressure from the boost will not get past the check valve and thus, cannot get into the valve cover and cause problems.
This is the engine bay of my SRT-4 (yes, it's filthy and it needs some attention). The green line starts at the top of the valve body and runs down to just behind the throttle body. The green circle is the vent that goes right back into the cold side intake pipe.
This is the result of too much boost. This is a picture of the cold side (pre-turbo). There is a bit of oil sitting there waiting to get sucked into the turbo compressor wheel. I actually cleaned this out last week (had a ton in there) so you can see what happens in just a few days.
As for some of our members who are seeing it at the throttle body, I would make a guess that boost is pressuring the valve cover and as soon as the boost goes away, the pressure goes back out the PCV (along with oil).
I've installed the catch can here with the line coming from the PCV and then going to the throttle body.The cylinder in the middle of the line going to the throttle body is the 1-way check valve.
The point is, turbo cars are a PITA, have ton's of vacuum lines every where and require special attention and upgraded systems when making modifications.
This catch can was $110 but catch cans can be had for $25 if you look. Very easy to install. I'll be getting one for the Cruze (not sure if I'll drop $110 on one of these, but maybe).
You're supposed to empty the can or replace it with each oil change.
I hope you find the informative. I'll do a specific write up for the Cruze when I get a system for it.
Please note that installing a catch can cause you to fail visual inspection when trying to get smogged.