Different from the existing fixed cam phase angle type, CVVT(Continuously Variable Valve Timing) is the device which varies cam phase angle continuously to be optimum. And with engine oil pressure, it operates. CVVT consists of OCV(Oil Control Valve) and cam phaser. OCV, mounted on cylinder head, controlls the amount and direction of oil delivered to cam phaser by oil valve which is connected to a solenoid. Cam phaser, rotating cam phaser rotor with pressure and amount of oil produced by OCV, rotates cam shaft forcefully for or against the rotating direction and finally, cam shaft phase changes. With the appliance of CVVT, engine power,fuel efficiency and the quality of exhause gas are improved.

Small particles in the engine oil may cause the oil control valve to bind or otherwise get stuck at certain spool positions. A test is used in this diagnostic to detect a stuck valve spool. A cleaning function is then used to try and free the spool. If unsuccessful, the diagnostic test is failed.

Figure 1. illustrates the principle of the valve stuck diagnostic test. As in the phaser error diagnostic illustration, there are two cases shown in the figure. The case on the left shows a case where the dutycycle is above a calibration threshold, yet the phaser position is near the minimum position. Under normal operation, such a duty cycle command would move the phaser toward its maximum position.The case on the right shows the opposite situation. The duty cycle command is below a threshold, yet the phaser position is near its maximum.

PCM monitors OCV stuck while cam offset is available and Valve cleaning is not in progress .If the PCM detects that CAM position angle is over 20 CAD(Crank Angle Degree) than expected cam positionthat PCM controls the OCV while cam position is in designated crank angle degree, PCM determines that a faultexists and a DTC is stored.