My mother-in-law made this cake. I don't have her recipe with me, but here is a very similar one that my mom uses.
1 pkg. (18-1/4 oz.) white cake mix
1 pkg. (3.4 oz.) instant pistachio pudding mix
1 c. lemon-lime soda
1 c. vegetable oil
1 c. chopped walnuts
In bowl combine the first five ingredients. Beat on medium speed for 2 minutes; stir in walnuts. Pour into a greased bundt pan. Bake at 350 for 35-40 minutes or until toothpick comes out clean. Cool on wire rack.