It happened twice.
For the first time, when I started it, at " parking " positon, the engine speed stayed at 1000-2000 r/min. After 2 min's warm up, I put the gear onto "Reverse" position, all suddenly, the engine speed went up to over 6000r/min. Then I turned off. When I tried to start it again, the engine speed remained at high speed of 6000. I had to turned it off. I tried several times, then I had to gave up. But after 30 min, I restarted it, it ran okey at parking position, like it should be. I did not try the reverse position.

A week later, it happened again. I warmed up the car for 2 min (let it running for 2 min at the parking position), and reversed out of the parking lots, and drove forward as nomal. 2 min later, I wanted to go back to pick up something, so I braked it down, stopped, and tried to turn the car back. I changed the gear onto "reverse" position. Then, the engine speed went up to more than 6000r/min again. I had no choice but turned off the car. I tried to started several time, still I didnot go back to be normal.
Then, I tried to depress the gas peddle before I started the car. After 2 times, it came back nomal.

So, anybody can tell me what is going wrong, is it because idle air controll valve?
Thanks!