Originally Posted by scottmcphee

Maybe your engine idle speed control circuit (that knob's wiring) has some poor connections, and the computer can't get a read on the setting. Or, the engine speed sensor is not picking up right. This is a closed loop feedback system, so the computer tries to compensate with speed adjustments until it can read the speed is right. If anything in the loop is wonky, it can't do its job, always hunting. But if you disconnect the idle speed actuator on the rear of the IP, you can tell if this is the problem or not. The car should drop to its lowest possible idle, and stay there. It might even stall if the engine is not fully warmed up. I forget what the number is, but it seems to me like about 550 RPM is the rock bottom.