Mercedes: RPM high about 20, rear muffler, vacuum leak

Question
I have 1989 mercedez-Benz 300E.
When I start engine, RPM stays at high about 20 and never drop
to 7.5 and after 10 minutes engine stop.
And engine crank but do not start. after waiting 1 or 2 hours,
Engine starts again but RPM stays at high about 20.
I have never had this problem before. One day rear muffler fell apart,without rear muffler, I driven 100 miles and then this High RPM problem happened. I fixed muffler but I have high RPM problem.
What is the problem of this high RPM?
I will appreciate your help.

Answer
Young,

The high RPM suggest a vacuum leak. I would begin to search for a cracked hose. There are short black coupling that connect the small plastic hoses.
There may be a possibility that when the muffler fell off it may have pull loose a vacuum hose under the vehicle. I would place it on a lift and see if that is the case.
Good Luck
Rob