idle speed fast

[944s2c]

1991 300e,drove home from work today,noticed my idle was @1200-1500 RPM.Found EHA to be leaking.Could this cause this problem?Disconnected plug from what I believe to be the idle control valve(bosch part no.0 280 140 150) and idle returned to normal.Reconnected plug,idle still normal.Turned off ignition,restarted engine,idle speed slowly went to1500 RPM.Swapped valve from another car ...same results. Measured voltage across plug appears to be 4.5 volts.Any clues as to where this valve gets its signal voltage???? Any tests to trouble shoot this problem would be greatly appreciated!