Air&Road, if the mass air flow sensor is bad the Check Engine light should be lit. I own a 1998 E320 with the V6 engine and it is very quick. As you mention an SLK320 should be as quick, if not quicker.

Surprised you didn't have your brother-in-law perform a PPI (pre-purchase inspection) and diagnose the problem before buying the car. That could have been used to negotiate a lower purchase price.

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Fred Hoelzle