Typically, if you see fuel in the line, that means that the diaphragm within the regulator has broken down and the reg should be replaced. Notice where the clear line leads to... the breather hose from the valve cover to the air cleaner. Fuel isn't supposed to be in that system. Too much fuel and you'll have idle issues or have a hot mess on your hands. It's a relatively cheap and easy fix to replace it with even a used one. Lots on ebay to find.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio