I would check and re-check all of your vacuum hoses, especially the ones going to and from the Idle Control Valve, as well as the other ends. Also, you might want to remove the Idle Control Valve to see if it is clean inside and partially open when no voltage is applied so air can pass through it. If it looks dirty, clean it with some carb. cleaner. You can test it to see if it operates by touching a 9v battery to the contacts.

Go over everything and report back to us.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible