On my ‘95 E300 there is a small puddle of fuel on the top of the fuel filter. I noticed that the black cap on top is a little loose.

Is it ok to tighten it snug or does it need to breath a little facilitate an equilibrium for the filter?

A little diesel fuel smell goes a looooooooong way

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles

Just from memory on my '91, there's sort of a flat headed bolt (natural silver metal color) that goes into the top of the big fuel filter. And yes, it should be VERY tight.