I heard about this on Clinton's car. He has a turbo-16v and had the same problem. You may want to look him up on this site and see how he overcame this.

It seems as though the air that is pushing through the throttle valve is holding it open because of the lever design. I think you may have to install a stiffer spring.

Can you reproduce the problem with the engine off? Or without boost? If not, it has to be that the area of the trottle plate is so large the mechanical force of the lever can't overcome it.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS