I had the same code on my w210 97 e320. I checked the vacuum lines for leak (no leak) and ended up changing the MAS. I was told on another board (or maybe it was this one) that after MAS replacement, I need to get the fuel trim reset by a MB computer. I haven't resetted the fuel trim yet and the check engine light came back with the same p0170 code after only a couple of drive cycles. The car drives fine except the annoying CEL.

delta
97 e320 with 57000 mi