How do I adjust the idle on an 02 chevy cavalier 2.2L V4 DOHC?

I have had people tell me to adjust the camshaft sensor, but the 2.2L has no camshaft sensor. I think the idle can be adjusted by using a computer, but I don't know what to hook up to the OBD 2 reader hook up to do this. The engine idles at between 500-600RPM and it should be at about 700-800 RPM. I just put high performance spark plugs in it, and changed the oil. The air filter is clean and the fuel filter is pretty new. It runs wonderful, just the idle makes the dashboard shake since the day I got the car. With the idle turned up to 700-800, the shaking stops. What should I check next?