There is a specific procedure for sealing the upper timing cover, and Hidalgo's link covers it.

When I replaced mine, I found the mechanic previously added sealant to the u-shaped seal. That's probably why it needed to be re-repaired with only 61,000 miles. I believe the seal needs to move (or float) in that joint. I lubed the top of the seal with a thin coating of silicone grease to help with installation since I don't believe in installing any seal dry. I have no leaks.

Use Loctite 5900 or 5999 (MB 002 989 73 20 10) to seal the vertical surfaces and a pea-sized dab at each corner.

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95 E320 Cabriolet, 169K