Test Groups WBMXV02.8M52 & WBMXV02.8Z35
General Description - Catalyst monitoring is based on monitoring its oxygen storage capability. Engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in exhaust gas. These oscillations are dampened by oxygen storage activity of catalyst. Amplitude of remaining lambda oscillations downstream of catalyst indicates storage capability. See Fig 1.
Monitoring Procedure - Conversion efficiency of both catalyst systems (Cyls. 1-3 and 4-6) are separately monitored on test group WBMXV02.8M52. On all models, to determine efficiency, lean and rich time periods of oxygen sensors both up and downstream will be compared. Smaller of two ratios determined is used for further processing. Cycle counter is incriminated by one count after each cycle. If counter reaches a predetermined number of checks and sum of all ratios determined is greater than a fixed value, a fault will be stored in computer memory. Monitoring procedure is then terminated until engine is shut-off and started again. See Fig 2-Fig 3 .