On your 90, you can run a diagnostic using the OBD1 port.
There are two modes, and you can check whether or not your idle speed solenoid (we call it Idle Air Control valve here) is working, and whether or not your car's switch is working to tell the car to idle.

http://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#FuelInjectionandIgnitionDiagnosticCodesi

And run the tests outlined in diagnostic test mode 2 and 3.

Mode 2 will check whether your idle switch works (tells the ECU to run the idle system) and mode 3 will test your IAC to make sure it moves. You'll hear a very distinct sound.

There's a lot else that can cause a crap-ola idle, but you can check this without disassembling anything or getting greasy.

Good Luck,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond