BMW Repair: 540I CHECK ENGINE LIGHT, gasoline vapors, purge valve

Question
I HAVE A 540I 1999 39K 8CY STICK WITH THE CHECK ENGINE LITE ON.  I RAN A PEAKE DIAGNOSTICS..ON IT AND GOT "PRECAT O2 SENSOR AGING,CY #5-8; ALSO MIXTURE CONTROL, OFF IDLE, CY#1-4 ; AND EVAPORATIVE EMISSION SYSTEM CODES.  I CHANGED THE PRECAT O2 SENSOR ..GAS LINE FILTER..AND CLEARED THE YELLOW ENGINE LITE AND IT REAPPEARED BUT THIS TIME WITHOUT EVAP CODE...THE CAR SEEMS TO RUN WELL...THE ONLY THING I NOTICED IS THERE SEEMS TO BE DECREASE PRESSURE RELEASE ONCE I OPEN THE GASCAP....HELP...ANT SUGGESTIONS...DON

Answer
Don, sounds to me like the Evap. Emmission's Purge valve is leaking or sticking open. Located either by the power steering resevoir or on the front of cyl 5-8 valve cover at front of motor. This two wire solenoid has to fuel vapor lines going to it. One feed line from the carcoal/carbon canister and the other discharge line from to the intake manifold. The valve is incharge of allowing or disallowing stored gasoline vapors from the gas tank via the carbon cansister to be burned by the engine depending on engine condition. It is actuated by the engine computer (DME). You should hear it making a ticking noise when the engine is warm at idle. Try clearing faults and changing this little valve. I don't know why an aging fault would be stored if you changed the driver side front 02 sensor.
-evan