The iac or idle air control is an electric valve that controls idle speed, if you look connected to the air intake hose before the throttle body there is a small( about 1"diameter) hose that goes down to a device below the intake manifold, and a hose comming back out of it to the throttle body. this unit controlls the idle speed of the engine, the throttle is essentially closed all teh way when your foot is off the gas, and the idle speed is controlled by the computer by opening and closing the IAC, if that thing gets sticky or gives out electronically, you won't be able to hold an idle, but will be able to keep it running with your foot on the gas.

Run the error codes as in the faq's and see what it says before spending any money on parts.
Also run diagnostic mode 3 where you will be able to hear and feel the iac opening and closing, as well as being able to hear the injectors firimng etc.
Wires and cap arent likely to all the sudden make it not idle.

It can be cleaned if sticky, not hard, bit if failure is electronic, would need replacement. Somewhere around here I do have the test procedure for that thing off the car., let me know if you need it
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale