There are a number of possibilities. To test properly you should monitor the signal to the idle valve with a multimeter or scope set on duty cycle. The valve is pulsed at a constant frequency with a variable duty-cycle. The system will go to between 70 and 80 percent (on) duty-cycle before it resets itself and starts over. The idea is that with the proper air going through the normal idle a midrange duty cycle will occur. If too much air is getting in due to partially open throttle or vacuum leaks, the system will put the valve in max duty-cycle trying to shut down the air to lower the speed.
So the key here is to see if the duty cycle is maxed out. If it is then the system is doing its best but the engine is getting too much air somewhere else. We use a smoke machine to verify the integrity of the manifold/vacuum systems.
Another thing to check is the throttle switch. The engine goes to a fixed open state everywhere but at idle. If the throttle is gummed up and not fully closing both things will happen: too much air and a fixed response.
I would also verify the power and ground circuits (for the idle controller) when the system is functioning (or mal functioning). I'm not sure but the Overvoltage Protection relay may power the idle controller and be suspect.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician