Idle speed to high Merc 230E

Idle speed to high Merc 230E 1996. Engine idling speed goes itself up to about 1.6 revs and never comes down. At start-up it stays just below 900 revs until I press accelerator to increase revs and after that it only goes down to about 1.6 revs and stay high. I consulted the garage ( Merc specialists but not Main Dealer ) they diagnosed the engine with their computer and advised me to change Mass Air Sensor ( which according to the computer was faulty ) I changed the Mass Air Sensor, but this did not help at all. Now the garage says "change the Throttle Body". I took off the throttle body cleaned it ant took the plastic cover off to check the mechanism inside. Mechanically everything there looks OK. The motor moves the butterfly valve when accelerated and comes back to it's lowest possible position. I am not experienced mechanic but my logic conclusion is that this device is working OK ( at least mechanically ). I checked for vacuum leaks, but everything looks OK. As an experiment, I took off the whole air conduct from the throttle body and left air inlet totally opened. The idle speed was exactly the same ( high ) as with all attachments fitted. I wonder if there is anybody on the forum who came across similar problem and may have some solution. Please help. Thank you for any suggestions. Mark, London