Rough idling can be caused by a number of things. If the engine's self-diagnostic system isn't of any help, then diagnosing the problem is a process of elimination. An engine requires three things to run: fuel, air and spark.
The idle air control valve is responsible for metering air into the engine while the throttle is closed (idling.) The valve's air passages can accumulate a varnish over time, blocking the passages or causing the valve to stick.
Clean the valve and the passages into the engine with carburetor cleaner, and reassemble. If this does not solve the problem, then the air metering was fine to begin with. Move onto the spark.
Depending on the number of cylinders your car has, an idle misfire can smooth out under acceleration. To test for this, set the car at idle and disconnect the spark-plug wires one by one, and listen for a drop in engine RPM.
There is no way to test for fuel system malfunction without a specialized fuel pressure test gauge. If you have one or know someone who has one, connect it to the service port and check for pressure variations at low RPM.
If none of these approaches produces results, then you will have to start replacing spark plugs and fuel injectors.
