In a Dutch oven, brown roast in oil. Combine tomatoes, chiles, taco seasoning, bouillon and sugar; pour over the roast. Cover and simmer 2 to 2-1/2 hours or until meat is tender., Remove roast to a platter and keep warm. For gravy, pour 2 cups pan juices into a saucepan. Combine cold water and flour; stir into pan juices over high heat until thickened and bubbly, about 3 minutes. Serve with roast.