Hi, i have a '96 W202 C280. After starting the engine and when the engine begins to get hot, the idle goes up and down. The engine is revving on its own. The idle becomes worse when the engine is hot, and a black smoke will come out from the exhaust. There is no check engine light yet on the intsrument cluster. Sometimes the engine will stall. There's no problem when i'm cruising, but when i have to stop or i'm in traffic, the engine revs on its own.I have replaced the fuel filter, cleaned the throttle body yet the problem remains.
Help!!

 

This sounds like a problem in the exhaust gas temperature sensor. The engine is getting a signal that is instructing it to go to rich mode then it finds it is too rich so leans off again. Then the cycle repeats itself.