Car: Audi S5 4.2L V8 FSI

I have my intake manifold out to clean carbon of the intake ports. When I was removing the port separators, I noticed some were really dry (light grey carbon), others were dark black (but not shiny). But I noticed two (circled) were black and shiny and looked wet. Is this something to be concerned about?

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  • Two questions: 1) What kind of vehicle are you working on? (make/model/engine) 2) You realize there are much easier ways of cleaning carbon deposits than taking all of this stuff apart? Performing a Seafoam treatment would have cleaned all of this out without any fuss/muss or posibility of destroying something in the disassembly process. Water under the bridge now you have it apart, but something to think about in the future. Commented Nov 19, 2016 at 13:09
  • @Pᴀᴜʟsᴛᴇʀ2 updated the description. This was 100k miles worth of carbon build up. It was really caked on, needed a pick and lots of elbow grease. I don't think seafoam would've cleaned up all the carbon to my satisfaction (I did check online first, common problem with this engine). Happy for you to prove me wrong. Thanks
    – tgun926
    Commented Nov 19, 2016 at 19:32
  • 1
    No worries! Since you now have it cleaned, doing a Seafoam on a 25K mile basis would make it so you wouldn't have to tear it apart again. For your issue, I don't know why it would be this way. Commented Nov 19, 2016 at 20:37

1 Answer 1


It seems to me that the only source for the carbon build up would be the PCV valve or maybe leakage from the intake valves, but given that most of the time air is being pulled in, I'd bet on the PCV valve. I'm thinking that what you're seeing reflects that pattern of air flow from the PCV valve outlet to the cylinders.

The wetness could be condensate from the PCV system, depending on how the car got to its current location (was it short drive without time to warm up completely) the wetness could be an artifact of not getting up to full operating temperature.

  • It was a spirited drive if I remember correctly (30mins). The PCV and throttle body is right near that part of the car, so I think you may be right. I was concerned it might be a leaky injector seal, but from the picture the injectors are below the intake port.
    – tgun926
    Commented Nov 19, 2016 at 23:41
  • If I understand what Audi/VW means by direct injection correctly I'm pretty sure that the injectors are actually inside the cylinder, like a diesel, so it would be pretty unlikely that you'd get fuel from them. You might have a cold start injector (old SAABs did IIRC) that goes into the manifold, but that seems unlikely… The PCV valve is the only logical source that I can think of, but the uneven distribution is a bit of a puzzle. Wise of you to be thinking about what could cause it while you've got things apart.
    – dlu
    Commented Nov 19, 2016 at 23:49

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