Hi everyone. Need help from all you sifus.

I currently drive my dad's waja most of the time, it's a 2002 Waja Premium 1.6X 4G18 Auto.

Every morning or cold start, the car will rev at 1,500 rpm, then it will slowly reduce to 1,000+- rpm after about 2, 3 minutes. I always thought cars with fuel injection no need to warm up, but when I reverse while the rev's still at 1,500, it will die! Help anyone? What could be the problem? Thank you very much