Scrape out what you can, it'll make the job a lot easier. Get a big tub and soak the manifold in a strong concentration of degreaser, it'll soften the crap so it comes out pretty easy with a bottle brush. Wear gloves, it makes a mess. If all else fails, Kerosene works well.

When you're done, disable the EGR system if you want to keep it clean.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]