Variable Camshaft Timing (VCT) System: Overview

The VCT system enables rotation of the camshafts relative to the crankshaft rotation as a function of engine operating conditions. There are 4 types of VCT systems.

Exhaust phase shifting (EPS) - the exhaust cam is the active cam being retarded.
Intake phase shifting (IPS) - the intake cam is the active cam being advanced.
Dual equal phase shifting (DEPS) - both intake and exhaust cams are phase shifted and equally advanced or retarded.
Twin independent phase shifting - where both the intake and exhaust cams are shifted independently.

All systems have 4 operational modes: idle, part throttle, wide open throttle (WOT), and default mode. At idle and low engine speeds with closed throttle, the powertrain control module (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. VCT systems provide reduced emissions and enhanced engine power, fuel economy and idle quality. IPS systems also have the added benefit of improved torque. In addition, some VCT system applications can eliminate the need for an external exhaust gas recirculation (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 the service information. For additional information, refer to the service information , Engine System - General Information. Verification of incorrect VCT phasing on a warm engine operating below 1500 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 the service information , Engine System - General Information.

PID	Description
VCTADV, VCTADV2, VCT_INT_ACT1, VCT_EXH_ACT1, VCT_INT_ACT2 and VCT_EXH_ACT2	Monitors the VCT advance and displays the advance angle in degrees. The actual camshaft position is measured using the camshaft position (CMP) sensor.
VCT1_F, VCT2_F, VCT1_INTK_F, VCT1_EXH_F, VCT2_INTK_F and VCT2_EXH_F	Displays FAULT or NO FAULT to indicate a VCT related concern is detected. The CMP circuit DTCs cause the VCT advance to default to 0. Correct any CMP DTCs prior to diagnosing engine timing or VCT DTCs.
VCTADVERR, VCTADVERR2, VCT_INTK_DIFF1, VCT_EXH_DIFF1, VCT_INTK_DIFF2 and VCT_EXH_DIFF2	Displays the error in VCT advance. The CMP signal is used to determine the difference between the actual camshaft position and the camshaft advance requested. The difference is displayed as a percentage that ranges from -5 to +5%. When the accelerator pedal is cycled this may range as high as 20%.
VCTDC, VCTDC2, VCT_INTK_DC1, VCT_EXH_DC1, VCT_INTK_DC2 and VCT_EXH_DC2	Variable camshaft timing duty cycle ranges from 0 to 100%. The PCM controls the VCT solenoid operation through the duty cycled ground.
VCTSYS	Variable camshaft timing system displays whether the engine is in open or closed loop. In open loop, the PCM defaults the VCT system to OFF (0% duty cycle). In closed loop, the PCM turns the VCT system to ON (varies the VCT duty cycle). If a VCT DTC is detected, the VCT system defaults to open loop operation.