engine cannot mainten idle
 

When I stopped at the traffic lights in this morning, my 94 C180 can't mainten idle speed and the engine stopped. When I restarted the car, I saw the engine rpm dropped from 2000 to 0 if I didn't step on the gas paddle. It seems the car cannot mainten idle speed at all. I tried to restart it several times but it didn't work. I opened the hood and pressed the wires and relays. The car then could start again. I drove it back to home directly. Does anyone has idea what's wrong with it? Any suggestions? Thanks a lot!