Idle Speed Control Valve (Stepper Motor)
WARNING: This page is about a different car, the 1995 Jaguar XJS, 1995 Jaguar XJR, and 1995 Jaguar XJ6. However, it is still accessible from the selected car via links, so may be relevant.
See IDLE SPEED & MIXTURE in ADJUSTMENTS - 4.0L 6-CYL article.