I have a 96 SL500. Today I replaced to engine vacuum hoses because they were all very brittle. These included the tubes running across from the left side to the right side, tubes from valve cover to air intake and small lines that run from the front of engines to the back. All appeared to go well. I have double checked and everything is connected.

Now, when starting the car, the engine idle speed starts out at 2000, drops a little and then accelerates to 3000 and never slows down.

Any ideas?

 

Sounds like you did not install the crankcase ventilation hose/restrictor valves correctly, allowing unlimited. crankcase vapors into the manifolds.

 

Is that the large hose that goes from the Valve cover to the air intake horn?