1993 Mercury Tracer 1993 Mercury Tracer Rough Idle

We just bought this and were told that it had carbon build up in the idler, we were told it was clened and they dont know whats wrong with it. It sounds like it wants to die But only when its in park or not moving. There are no dash lights on. But it is idling very very rough and turns off if you let it, and wondering what it could be. It will stay on if you keep your foot on the gas, but as soon as you let go the rpms will drop, and in attempt the car tries to bring the rpms up, but it still doesnt work.