Did your mechanics check your throttle switch. Some of the world models had a vacuum switch on the left fender to gauge the idle position. Most other models had a switch on the throttle shaft. If this switch is open the car will idle high.
I would also test for vacuum leaks. I would also test the idle valve to see what its doing. The early systems could close the valve down with a 70/30 duty cycle. This means that the valve is pulsed with a steady frequency (say 100hz - 100 times a second). The pulsewidth is then 1/100 of a second. For this period the coil can be energized 70% of the time and off 30% of the time. This is as far as the system can energize the valve. The opposite to this would be holding the circuit for 30% and off 70%. All proper idle correction lies in between. So... one must measure this to know whether the system is trying to do its job. If the system has the valve energized 70/30 then there is either too much air entering from other sources or the valve isn't being closed by the strong signal.
To verify whether all the air is coming through the valve the passage can be blocked; the engine should die.
I would also be sure your timing isn't too high.
Bosch Master, ASE Master, L1
33 years MB technician