Fumes in the car W220

Hi, I'm new to the community ?
I have recently purchased a 2003 S320 (W220) it has the "Black Death" smell in the cab, I fixed this problem, but there is also another fume type smell, I removed the oil cap today whilst engine was running and a lot of smoke/ backpressure was present, also the breather thing on top of the cam cover there was smoke coming out of where the cover clips together (not Seal related), I think this smoke is the smell I can smell inside the car, it seams to be some sort of backpressure issue, I can't understand how the smell is so strong inside the car but hardly smell it under the bonnet or outside the car, could engine elaborate or had any experiences with this? I've checked all the pipes etc for tightness and seams fine, also cars giving out a fair amount of black smoke under acceleration.
Sorry about the long winded post.
Thank you