While I'm far from knowledgable I might suggest that you find a shop in your area that has a "smoke machine". That is the only way I found my vacuum leak(s) after weeks of searching. The test was $100.00 and I wish I had done it weeks before!

http://www.peachparts.com/shopforum/showthread.php?p=1920679#post1920679

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82 380SL 41K miles sold, 98 SL500 44K miles, 13 C300 4Matic, 14 CLA-250 4Matic