Expert: Don Berlan - 9/3/2009

Question
MY YELLOW CHECK ENGINE LIGHT HAS CAME ON..I'VE RAN A DIAGNOSTICS AND IT GAVE FAULT CODES OF BANK 1 SYSTEM TOO LEAN AND BANK 2 SYSTEM TOO LEAN.  I NOTICE THE O2 SENSORS FLUCTUATED FROM LEAN TOO RICH PRECAT BUT NOT POSTCAT....THOUGHT THIS WAS THE PROBLEM & CHANGED THESE POSTCAT SENSORS OUT..THE CAR RUNS ROUGH MOSTLY WHEN IT FIRST STARTS BUT GOES DOWN THE ROAD WELL...IT HAS 42K MILES..I'VE CHANGED THE EGR VALVE WITHOUT HELP...I PUT SOME INJECTOR CLEANER IN & IT MADE THE CAR RUN MUCH WORSE WONDERING IF I WOULD END UP STRANDED...THIS ONLY HAPPENED FOR 2-3 MILES & GOT BETTER AGAIN...ANY SUGGESTIONS..MORE HX OF THIS CAR IT'S A SUMMMER CAR & STORED WINTERS SO THEREFORE WHERE I LIVE THE WINTER'S ARE EXTREMELY COLD WITH SNOW..THX DON

Answer
Well you had the codes but I believe you changed the wrong sensors.The post cat sensors do not fluctuate as the front do.That means they are working.The rough running when cold would also point toward the front sensors.They do get more use than the post cat sensors.Check over all the engine systems for any air/oil leak,the exhaust for any leaks as well.Going back over the question I am lost for what year,model of BMW. Model years 1996 and  later have obdII earlier years don't and diagnosis is ALOT different.Fuel pressure should also be checked and the filter changed especially since it sits for the winter months.Although it has low mileage changing the filter is important due to sediments settling.