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.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator