Mercedes tank systems hold some pressure (probably not much, 0.5 psi or less) as an emissions control measure. That's what you hear when you open the tank. Vacuum isn't supposed to occur.

I've also heard of spiders, the kind that make or use tunnels, plugging up BMW tank vents. MB tank vents don't seem to attract mud daublers too badly, but it does happen.

They also like weed whacker exhausts, as I found out the hard way a couple years back!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!