Fast idle
 

Hello everyone need some help with the duramax engine was driving normal on interstate the motor clattered then when stoped notice was idling high the rpms bout 900 and it started shifting weird up and down like trans slipping feels like it is trying to push threw red-light Now when starting it revs up and stays if mash pedal it revs up then when let off it surges then back to high idle Does anyone have any ideals on whT it could be and how to fix. Thxs in advance for time and info


Trk is 01 chevy 3500 2wd