Theory Of Operation

One aspect that aids in accelerated catalyst light-off, for emission reduction, is performed by phasing of the camshafts following an engine start at cold conditions. During a cold start, while the engine is at idle, intake and exhaust camshafts are actively phased in attempt to reduce cold start emissions. The camshafts are controlled independently by the use of solenoids that phase the camshafts, advanced or retarded. A separate Variable Valve Timing (VVT) diagnostic is used during this cold start period to determine if the camshafts phasing is being performed as intended. The diagnostic has its own modeled camshaft position. This modeled position is compared to the actual camshaft position. If the average difference (error) between the actual and modeled camshaft position over the diagnostic test time exceeds a threshold a fault will be set. The diagnostic evaluates each camshaft independently and in both directions, advanced and retarded.