And now you have a car that emits more NOx than ten VW’s and is illegal in all 50 states and Europe. Every 200k miles, clean the manifold and forget about it. Gasoline can be used (with care) to wash that stuff out.

Little known or noticed: your engine has a PCV valve. In another example of brilliant German engineering, it’s built right into the valve cover and is non-replaceable. I’m pretty sure this accounts for more than it’s fair share of manifold sooting. Search for my thread on “valve cover, dissected” to see how this gem works. I’m still looking for a way to repair it.