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Car: 2008 Audi S5 4.2L

I found an oil leak at the bottom of my car near the engine/trans, which lead me to believe the rear main seal. However, one mechanic told me it was the timing cover gasket, while another told me it's the valve cover gasket.

I had a look at the engine bay from top and it looks like there is quite a severe leak, however I'm not sure if it's the valve cover gasket, or the seal under the guide frame (part 11, valve cover sits on top of it):

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Here's a picture of what I saw (the top gasket is between the valve cover & guide frame, bottom is between guide frame and head). Also note the huge mound of gunk behind the plug:

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Here's a picture after I washed it down with degreaser:

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Became liquid naturally falls down and back when accelerating, I think it is in fact the valve cover, and not the seal under the guide frame. However, I'm not sure. What are some signs that it is the valve cover gasket or not?

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You are right about leaks in that the liquid will flow down from where it leaks at. In your case it is really hard to tell exactly where it's coming from, but would suggest you are right about the valve cover being the source. It usually is a primary source for most leaks of this type. Cleaning off the area was a great first step in figuring this out. With one more thing, you can be proof positive where the leak is coming from. That step being, adding some fluorescent die into your oil which will show exactly where the leak(s) is/are coming from. Here is a representative product which would help you:

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You'd also need UV glasses and a light to assist, but this should make the leaking area stick out and become quite visible. The die itself (as shown) is quite reasonable. You can get a kit with everything you need (die, light, glasses) for a relatively cheap price as well. If you have an older car where leaks may become more common, getting the kit may not be a bad idea to help you out in the end. The die will not harm the internal workings of your engine, so have no fear there.

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  • I didn't buy the magnetic dye, but replaced the valve cover gasket. This may be hard to answer, but roughly how many KM should you put on the car to verify it's still not leaking?
    – tgun926
    Sep 19, 2016 at 21:47
  • @tgun926 - If you have fresh oil in the engine, it will be hard to see until gunk starts building up ... thus the reason for the UV dye ... it really does help. You're right, very hard to answer, but would assume many, many miles without the dye in it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 19, 2016 at 21:55

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