I just went through my '91 420 SEL. All the soft rubber bits had turned to hard plastic. The little connecting plastic tube on mine is acutally crimped shut on one end. It's made that way. I looked up the part no. on the Russian site then did a Google search for parts and sure enough it's suppose to be that way. On other models and years it is a hollow tube.
Mine was really running rough and surging at idle. I also removed the intake manifold, replaced the eight rubber connecting rings between the manifold halfs, and the big rubber piece that connects to the throttle body. All toast. Inside of intake had major carbon and coke deposits. Replace the fuel injectors, holders, and O-Rings. Replace all vacuum lines and rubber ends. Runs very smooth now, just idles high. About 1100 rpm.
Let me know if you solve your idle thing.

Dave