You'd probably be better off asking in the Performance Paddock section, as well as digging around on http://www.msruns.com in the Daimler Benz category for information on similar cars.

Just off of the cuff, I'm guessing that you don't have anything controlling the stock idle air valve, and it's defaulting to full open, thus giving you the 1000rpm idle.
If the valve is anything like the m117 engine uses, it's a PWM valve that is full open at 0% duty cycle, and full closed at 100%.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...