Variable Camshaft Timing (VCT) System

The VCT system enables rotation of the camshaft relative to the crankshaft rotation as a function of engine operating conditions.

The VCT system has 4 operational modes: idle, part throttle, wide open throttle (WOT), and default mode. At idle and low engine speeds with closed throttle, the PCM determines the phase angle based on airflow, engine oil temperature and engine coolant temperature. At part and wide open throttle the PCM determines the phase angle based on engine RPM, load, and throttle position. The VCT system provides reduced emissions and enhanced engine power, fuel economy and idle quality. In addition, some VCT system applications can eliminate the need for an external EGR system. The elimination of the EGR system is accomplished by controlling the overlap time between the intake valve opening and exhaust valve closing.

The VCT system knocking and noise concerns are diagnosed in Engine System -- General Information article. Verification of incorrect VCT phasing on a warm engine operating below 1, 500 RPM can be isolated using a stethoscope and by monitoring the PIDs using a scan tool. If the VCT phaser does not maintain correct valve timing, low oil pressure or oil flow restrictions are primary possible causes. Verify correct oil pressure and flow, refer to ENGINE SYSTEM - GENERAL INFORMATION