usually the tube gets cleaned; the valve may have to be changed. do a search in the ml forum-the ml320 had the same engine & the techniques are described there.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)