action tank_controls()
{
 ....

    if(key_e)
     {
       start_stop_eng();
       while(key_e) wait(1);//this will work but action will hang until user stops holding e.
     }

}

action tank_controls()
{
    var e_on_last_frame;
 ....
     while(.)....
         if(key_e && e_on_last_frame==0)//this will only work once if player holds e
          {
            start_stop_eng();
          }
          //just before the wait
          e_on_last_frame = key_e;//this will let you know if e was being pressed on prev frame.
          wait(1);
     }
}