My tachometer slowly but steadly went to 9,000 RMP. Now the tachometer constantly reads 9K even when the car is totally shut down.

Any suggestions on what it causing the problem?

 

your tach is either broken and you need to replace it or there is an electrical current that is running through your tach that is making it inaccurate (a lot of stereo equipment, or the connecting wires behind your tach may need to be resodered.