Ironic, because I got a call from work today from my brother saying the 300E (1987) was dripping (from somewhere) and smelled like gas. Turns out he's an idiot and either lost the gas cap or someone stole it. One more thing I need to fix when I take it back
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I'm going through the entire fuel line in my head. And maybe I'm missing something, but your could try replacing you fuel cap. May not be allowing enough air in. Thus causing the pressure in the gas tank.
I know I didn't answer your first question, but I'm at a lose. A MB tech should be able to figure it out. Unless it something as simple as a gas cap
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I'm no MB Tech, but I drove one for years.
Good luck