You could try running a Qt. of Xylol / Xylene from home depot (or almost any other hardware store) paint dept. in about a 1/4 tank of fuel and giving it another go.
my buddy lives in San Diego and he has a 85 380sel that he has to do that to, to get it to pass. its a cheap trial.

Just a thought
Good luck

David Lee Portland OR
87 300D
87 190E 2.3
84 230E Euro