Vacuum line under intake to throttle body?
 

I found the same issue with both my 1991 300SL and my 1990 300E last night. A vacuum line that connects to a small nipple under the intake manifold just to the left of the throttle body (you have to look hard to see the nipple just to the left of the rubber boot connecting the throttle body to the manifold) was disconnected.

I went ahead and used a wire tie to bind it to a rigid tube next to the line along its path to the nipple so that it stays properly aligned and to eliminate any tension in the line.

What is the purpose of this vac line? Now that it is connected, what difference should it make?

Thanks,
AL

I think there are 2 nipples: one is capped off and one is used for measuring air into the intake I think.

It is where the engine pulls in fuel vapor from the gas tank. When its disconnected you occasionally get a whiff of fuel and risk a fire.