TortugaTortuga

Noticed a fluid build up (presumably oil) on the topside of the engine bay around the intake manifold hose and above the oil filler tube (picture below) in a 2006 C4S w/ 70,000 miles. What's causing the leak? Any cause for concern? Thanks for you help.

semicycler

Could be combination of an AOS going bad putting oil into the intake, and a loose band clamp on the intake plenum creating an air leak (along with the oil that shouldn't be there).

Are you getting excessive smoke on startup or while driving? Is the oil cap difficult to remove while the engine is running due to excessive crankcase vacuum? Is there oil in the intake plenum (pull the airbox out and perhaps even the throttle body to check)? These are some signs of an AOS going bad.

qikqbn

The AOS recirculates oily vapors back into the plenum. The oil/vent tube just to the right in the photo is one of those passageways. I would not worry too much about this, but as semicycler asked... are you noticing any smokey startups? That could mean an AOS is not working as efficiently and replacement is needed. If not, then just a good cleaning would do the trick. Nothing to freak out about. I have seen this on my car too in the past. When I do a throttle body cleaning I also take some extra time and loosen those clamps, move the rubber connectors on the plenum just enough to get a good cleaning around them. Then tighten down those clamps again. It inevitably will seep a bit of oil again, but like I said, nothing to worry to much about. Just keep and eye on it so that it does not get excessive.

lmaternick

Be very careful if you decide to tighten the hose clamps on the resonance tube or intake plenum. The rubber boots can split causing a vacuum leak. these should be snugged up with a screwdriver....not torqued down tight with a ratchet and socket. Those band clamps are sharp. Because these are under vacuum when the engine is running I am doubtful that your oil is coming from there unless you have a split rubber already and a lot of oil residue in your intakes. I am in the middle of taking all that stuff apart and cleaning things up. As I have done so I have noticed that the area being coated in oily grime is right above the connection pipe for the left side oil separator where it attaches to the AOS (indicated where I am pointing to disconnected port). These connections are pretty crappy design and may not be "clicked" fully into place on the AOS. I have cleaned mine up but it looked exactly like yours. I had no AOS failure symptoms. In the photo I am pointing at the tube connection that wasn't fully engaged. In the second photo I'm showing the o-ring (orange) that could be damaged or not sealing properly. Any of the connections at the AOS could be suspect so you will want to disconnect all that you can around the oily area and confirm that the o-ring is good and that it fully engages back into the AOS when reconnecting. It will lock into place if it is secured correctly. Push it on and then try pulling it off. If it isn't locked in place it isn't good enough. Check those that you can reach easily...I wouldn't bother disturbing the hard to reach connections as you could make more work for yourself than required. Really you have the two large ports on the left side that are directly in the problem area and are relatively easy to reach and check. I do notice that you have your plenum and throttle body still in place. This will make it difficult to reach the AOS. If you are capable the plenum is easily removed but you'll have to remove the airbox and throttle body to get to the plenum. Otherwise just clean it up carefully and live with it.
Bye the way I was going to make the exact same post and ask the same question about a week ago, before I decided to dig deeper.
Hope that helps.