The vent valve on the 124 is designed to hold a certain amount of Vacuum or pressure, i have looked it up before for a similar question,dont have the cd at hand right now.At least for that model the vent is not open to atmosphere,yours may be the same or not. Found it,normal operation pressure up to 50mb or vacuum of 1 to 16mb.the pressure relief control is in the fuel cap,the vacuum control is accomplished by the vent valve.Don

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97 E300D 157000 miles
87 300TD ?141k? miles