I was cleaning up the battery tray in my 92 300TE, and found a vacuum leak that was well-hidden, but may be common to these cars.

Under the battery tray is a vacuum line that is only partially shielded by a black plastic hose. While cleaning out the accumulated crud in there, I noticed that there was an "unprotected" section of vacuum line (running into the battery area from under the wiper motor area to the fender) that had developed a 1mm hole in the side of it. I suspect that over the last 14 years, neglected batteries spewed junk down onto this vacuum line, and it eventually developed a hole.

Cut the vacuum line at the hole, and use a stiff wire to pull out the gunk that has gotten sucked in there. Use a 3cm piece of vacuum connector hose to reconnect the line.

I don't know which engine component uses this line, as it disappears down into the fender under the hood hinge.

Hope this motivates someone to clean out under the battery tray in a W124 (good PM) who is also dealing with vacuum leaks.