??? You mean the one at the front end of the cylinder head that pushes on the long chain rail, right? I see it from our site hosts and other online sources when I search for chain tensioner. Clean the area then check if it's leaking from the cylinder head mating surface or tensioner body halves. There's a crush ring (called gasket) between tensioner and head. I don't think there's one between body halves but similar tensioners come apart for resetting and internal cleaning.

Sixto
98 E320s sedan and wagon