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As was already mentioned, completely pinch off the hose from the idle air controller. If the engine slows down you know the idle air controller is open. If the engine speed stays the same the throttle is being held partially open.
-- DISCLAIMER: The following text applies to a '90 240 with LH-Jetronic Fuel Injection 2.4, and may or may not be applicable to your model--
The idle air controller will be partially open at rest (without any signal from the computer) by default. This is proper design as it will prevent the engine from constantly stalling if the signal to the controller is lost for any reason (bad wiring, loose connector, etc.). It could very well be that the idle controller is not getting a signal from the computer, and the controller is in its resting position (partially open).
One other thing to check is the throttle switch. With the engine off, slowly turn the throttle open. You should hear a distinct 'click' just off idle. If you don't, the throttle switch may need 0adjusting or replacing.
After hearing the click, check the switches with an ohmeter. There should be two switches in the same housing; one for idle and the other for wide-open throttle. The center contact is the common for both switches. The idle switch should be closed at - you guessed it - idle.
If the computer doesn't see the idle switch, the computer assumes the engine is now under manual control and turns of the idle speed 'auto-pilot' since it is no longer needed. A maladjusted/malfunctioning idle switch could account for the problem you are experiencing.
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