Variable Valve Timing (VVT) System: Avalon
The VVT system controls intake camshaft valve timing to provide improved engine performance and fuel economy, and reduce exhaust emissions. VVT system consists of 2 camshaft timing oil control valves, variable valve timing controller on each intake camshaft, crankshaft position sensor, engine coolant temperature sensor, Engine Control Module (ECM), variable valve timing sensors and associated wiring. See Fig 1.
Exhaust camshaft is driven by the timing belt and intake camshaft is driven by gear on the end of exhaust camshaft. Intake camshaft drive gear is integrated with the variable valve timing controller to vary intake camshaft valve timing. Variable valve timing controller consists of a housing driven from the exhaust camshaft and a vane that is fixed on the intake camshaft. See Fig 2. Oil pressure may be delivered from the advance or retard side of the intake camshaft to the variable valve timing controller. This oil pressure causes variable valve timing controller to rotate, causing intake camshaft to rotate and change the valve timing. When engine is stopped, to improve stability, intake camshaft will be placed in the most retarded state. At this time, a lock pin secures the housing and the vane inside the variable valve timing controller. After the engine starts, lock pin is released by oil pressure.
Camshaft timing oil control valve is an electrically controlled valve that receives oil pressure from the oil pump. See Fig 3. The ECM uses input signals for engine speed, intake air volume, throttle position and engine coolant temperature to determine operation of camshaft timing oil control valve. The ECM also uses input signals from variable valve timing sensors and crankshaft position sensor for determining the actual intake camshaft valve timing. Variable valve timing sensors may also be referred to as camshaft position sensors. The ECM operates camshaft timing oil control valve by controlling position of spool valve to determine which side of variable valve timing controller the oil pressure will be applied on for advancing or retarding the valve timing by rotating the intake camshaft. See Fig 3. When engine is stopped, camshaft timing oil control valve is in the retarded state.
When engine is idling, intake camshaft valve timing is set at the standard or hold position to stabilize the idle and obtain better fuel economy. During light engine load, intake camshaft valve timing is retarded to provide stable engine operation. During medium engine load, intake camshaft valve timing is advanced to provide increased fuel economy and improved emission control. During heavy engine load in low-to-medium-speed range, intake camshaft valve timing is advanced to provide increased torque. During heavy engine load in high-speed range, intake camshaft valve timing is retarded to provide improved high-speed range operation and better fuel economy. During cold temperatures, intake camshaft valve timing is set at the standard or hold position to stabilize fast idle speed and obtain better fuel economy. When engine is started or stopped, intake camshaft valve timing is set at the standard or hold position to improve engine starting.
If a problem exists in the VVT system, Diagnostic Trouble Code (DTC) may be stored in ECM. See SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article for retrieving and servicing of DTCs.