Two common problems are the gas cap and the EHA, which is attached to the fuel distributor under the air cleaner. If you can see it leaking, replace it.

After that, I would look at the rubber fuel lines in the fuel pump assembly.

And yes, don't top up the gas tank. I one time topped up the tank and drove home to add a bottle of Techron. When I removed the gas cap, about a pint of gasoline came shooting out.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino