A little bit is not a problem. A giant whoosh is bad.

The key is to check again if fuel will properly drain from the tank once the filler cap is off - if not, you likely have a blocked fuel strainer. The typical reason for strainer blockage is fungus/algae.

You can search on algae here and get a lot of information. The key is maintaining a water reduction & elimination strategy, most easily done with a fuel additive like Power Service. And keeping the tank nearly full in really cold weather.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles