What year is your 300E? Your throttle linkage may need to be adjusted, or your deceleration shutoff microswitch replaced, among other things.
I would check these first. When your engine is idling high, press the little roller wheel in on the microswitch. Your throttle linkage makes contact with it and tells the engine to reduce fuel inj. If the idle decreases when you test it, then you can try to bend the little roller arm so it pushes farther in, or just replace it. Dealer list cost is $23.
Other things to check:
air flow sensor position indicator
throttle valve switch