On the 300E the idle would be at 500RPM or less if the overvoltage relay was bad.
There are four sensors involved with correct idle speed on this car. Sounds like at least one isn't sending correct info the CIS-E computer.
These sensors include idle contact, decel swittsch, air flow position sensor, & hall-effect speed sensor.
A test of these imputs to the ecu will lead to correct diagnosis & repair.
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190E 2.3 16V ITS SCCA race car (sold)
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