I may have found it. I got home today and checked with the engine hot. As I ran my hand down the back side of the left valve cover, I found enough oil to drip off my fingers under the lower edge of the rear camshaft cover. This is a plastic "plug" that is on the back of the left valve cover (where the centrifuge is on the right valve cover). I removed it and I see that the o-ring is very flat. I'll get a new cover and see if that fixes the leak. The problem is, I can;t find a part number online or in EPC/WIS. I guess I'll just go to the dealer. I can't imagine this little piece of plastic could cost very much (yeah, right, famous last words!)
CLK550 Mystery Oil Leak
Almost every day, I find a small puddle (maybe a teaspoon) of oil under the car. It's basically right under the left side catalytic converter. It seems to happen after driving. I've removed the bottom engine covers and tried to find the source, but I can't. I see it on the steering column universal joint and the left cat (I sometimes smell it on the cat while driving). I see no signs of oil around the valve cover or even the head. I think I might see traces of dampness around the top of the exhaust manifold, but it's not wet enough to be leaking this much. Any ideas?
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OK, so this is now resolved. The part is called a "cam plug" and there are two of them on the back of left-bank valve cover. My leak was on the larger one (about 2.5" in diameter) that's behind the intake camshaft. There is also a smaller one (about 1" in diameter) behind the exhaust camshaft. Both of them together were $14 retail. I went ahead and replaced both. The guy at the parts counter told me it is a very common problem, and the symptoms are exactly what I had with it being very hard to find the source of the leak.
FYI "left" side = USA driver's side. And yes, you can use a mirror, but really, if you'll just use your fingers - run them down the back side of the valve cover. If they come back oily, it's 99% the cam plugs. They don't screw off - they just snap in place. Use a plastic screwdriver to pop them out.
If you remove the plug and look at how it's made, you'll understand why it leaks. It uses a simple o-ring molded to the plastic cap) for a seal. With the heating and cooling of the engine, it flattens out, expands then shrinks, and leaks. The leak will just get worse. Go buy new ones and snap them in place -it's cheap, easy and will stop the leak (at least for a few years).
A0009985590 is the larger one for the intake cam, A0009985590 is the smaller for the exhaust cam. I believe those are the latest part numbers, but your parts counter should know for sure.
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