Misfire code for #4 clyinder
 

I have a 97 C280 in the shop that has a misfire code for #4 cylinder and multiple misfire codes. I've replaced the spark plugs, #4 ignition coil and connector and I still get the misfire code for #4. One thing that puzzles me is a humming sound coming from the rearmost rails of the intake manifold. Could the resonanse flap be causing all of my problems? Another thing the MAS is reading 1.28 which I believe it's trying to rich the mixture so would that point to a vacuum leak. Any help please. I've never seen this problem before.

Sounds like you have either a vacuum leak at #4 cylinder OR a "weak" #4 cylinder & a bad mass air flow sensor.