That high pressure line has been known to leak. It happened to my old 1993 400E and my 1994 E420. I noticed that when I changed the fuel filter a few days ago on my 1995 E420, the prior owner had already replaced the line. Another guy over on the 500E board just had to replace one on an E420 he just bought too.

I had ordered the line in anticipation of it being bad. It was only about $30 for the genuine Mercedes part.

If you change the filter, there are two copper washers in the front of the filter where the banjo bolt connects. It would be cheap to order these with the line and filter rather than take a chance reusing the old crush washers.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles