I am having rough idle issues with my just purchased 1999 MB C280.Previous owner mentioned that Regular Gas was used (full tank),since then the problem started.... Codes P0300 & 0303(multiple cylinder misfire & cylinder 3 misfire) come up with Scanner!! Any help/advise as to what I need to check for would be appreciated!!.....Thanks.

I don't have that engine so I don't have specific experience. I don't think a tank of regular would do that. When mine had code 0302 or 0304 it was a bad coil one time, and bad plugs (I think the other time). There are also connectors between 3 of the coils and the plugs which people say go bad, but mine didn't seem bad. However, the multi cylinder misfire code kind of seems to point to something more general. Hopefully someone else has a better answer. Did you search on code 0300?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Code 0300 is Random cylinder misfire & 0303 is #3 misfire. I believe,it may be coil issue,MAF Sensor &/or ???....Checking on BenzWorld past posts related to C280 Rough Idle,seems to be common MAF Sensor problem with C280....I hope members give some more input before I start process of elimination to correct the Rough Idle & Code problem............Thanks.