The early idle control systems would go wide open when unpowered. You should get at least 1500rpms disconnected maybe over 2000.
A cheap fix that I have seen more than once is to cram a broom handle into the idle air hose and drill a 4mm hole through it. This would give a slightly high idle, warm, but a low idle cold (as the same volume of airflow produces different speed at different temps). BTW, broom handles are only one off many types of restrictions people use to shut down the airflow when the electronics fail.
Bosch Master, ASE Master, L1
33 years MB technician