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What grade IMU are you using? If you only give the ekf angular speed around the vertical axis, it will have to integrate it to deduce yaw. Any small bias/error in angular speed is going to grow really fast since you are squaring angular speed to get the yaw. Rather than feeding angular speed to the ekf, try to give it the actual yaw from an electronic compass.

Also, unless the robot is perfectly flat, you will have issues with gravity interfering with linear acceleration measurements. Robot_localization can remove gravity, but for that it must know the pitch. In my experience, using acceleration to deduce change in position (unless using extremely high grade accelerometers) is a waste of time. You're better off relying on the wheel odometry for that.