Not a lot of help here, but i'd build a code reader and see if the ECU, EA/IS/CC, or emissions control is throwing any codes that could lead you in the right direction.

Heres a list of codes and a code reader schematic in a different thread:

http://www.peachparts.com/shopforum/showthread.php?t=107630

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE