Have 94 E420 with the dreaded rough idle. The following parts have been changed air mass meter, fuel pressure regulator and the throttle body module and it still idles rough. I did notice that the check engine light does not work at all. The car has been to an independent shop, mechanic was a mercedes shop tech for 25 plus years. When he hooked his diagnostic tool up the car, received the following message 'Check the connection could not communicate with the computer.' Last option is to take it to the $dealer for diagnostic check. Please note the car was brought used don't know when it was tuned up last.