In another thread someone suggested that the source to a problem I am experiencing might be because the supply & return fuel lines on my '87 300D may have been swapped at the metal lines on the chassis. My question is which line is the supply and which line is the return line. I did notice that one of the metal lines has a dot of red paint on the line. My original question was prompted because the clear in-line fuel filter is only about 50% full of fuel. It appears that air is getting into the fuel system. For some reason the filter is never more that about 60% full of fuel. My other diesel always has the filter almost full with just a small air bubble. I would like to rule out the question about the lines being switched. How do I determine which line is which? Any other thoughts?

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• 2001 E320