The water supply tube that is mounted to the underside of the upper rack on this Whirlpool dishwasher has been re-designed. Therefore, just puchasing part #3378144 will not work. You need to purchase the entire upper spray arm kit.
If your current tube is made up of three parts, an extruded tube with a molded funnel on one end and a molded elbow on the other, you are in the effected group.
Good luck