1995 Jaguar XJ6 stepper motor on throttle body

Changed out 95 engine with 150,000 to a 96 engine with 13,000. Idle r.P.M. 2000 to 1800. Took throttle body off cleaned, changed stepper motor with 95 engine. Extended piston on stepper motor to ensure proper seal before tighting bolts. Eninge idled at 250 to 300, adjusted idle to 500. Once I raised throttle r.P.M. With peadle, r.P.M. Raised idle r.P.M. To 1200. Stepper motor acts like piston not fully extending.
What would your next step be