I recently got an old 2009 w204 c220 cdi 170hp manual with about 200k km, it initially came with a hole in the intercooler tube, some burnt cabling around the fuel injectors and one of the injectors has been recycled or whatever the word is in english, but everything has been fixed and replaced in the car and its as fresh as new.
Problem is when starting the car after half a day of sitting, even in 20-30 C° weather it starts revving up on random until it warms up to 60+, it initially stays at about normal idle rpm then jumps to 1.1k-1.3k at random intervals and at random strengths, its sometimes a 100rpm jump and sometimes 500. Ive looked around google and some say its a missfire, some say its idle hunting and some say its normal when cold so i dont know what to do.

 

Hey! Sounds like you've put a lot of work into getting that W204 C220 CDI back in shape. The revving issue you're describing could be a few things. Since it's happening when the engine's cold, it might be related to the air-fuel mixture adjusting itself as the car warms up. This could be normal, but given the history of the car, it might also be worth checking the idle control valve or even the MAF sensor. Sometimes, sensors can get a bit finicky, especially with older cars. It could also be something simple like a slight air leak somewhere in the system. Keep an eye on it, and if it persists, a diagnostic scan might shed more light on the issue. Good luck!