Variable Camshaft Timing (VCT) System: Overview

The VCT system enables rotation of the camshaft(s) relative to the crankshaft rotation as a function of engine operating conditions. There are four types of VCT systems.

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

All systems have four 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 air flow, 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. Currently, both the IPS and DEPS systems are used.

The VCT system knocking and noise concerns are diagnosed in the appropriate Service Information. For additional information, refer to the appropriate 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 VCTADV, VCTADVERR and VCTDC 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 appropriate Engine System - General Information .

PID DESCRIPTION

PID	Description
VCTADV	Monitors the VCT advance and displays the advance angle in degrees. The actual camshaft position is measured using the camshaft position (CMP) sensor.
VCT FAULT	Displays YES or NO 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	Displays the error in VCT advance. VCTADVERR uses the CMP signal 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	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.