im not exactly sure what you mean about 'flatness', but i guess you mean acceleration. that car should be pretty quick (for a diesel). 0-60 in about 10 seconds. could be just losing boost, but who knows.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350