Variable Camshaft Timing Operation
WARNING: This page is about a different car, the 2017 Jaguar XJ, 2016 Jaguar XJ, 2015 Jaguar XJ, 2014 Jaguar XJ, and 2013 Jaguar XJ. However, it is still accessible from the selected car via links, so may be relevant.
When the engine is running, the compression and expansion of the valve springs causes momentary increases and decreases in the torque acting on the camshafts. These momentary changes of torque are sensed in the VCT units and used to change the camshaft timing.