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BMW 325IS failed smog check due to high NO at 15mph.

1992 BMW 325is failed smog (15mph NO) in 2008 & 2010. My '92 BMW 325is with 150K miles failed smog (high NO @ 15mph) in Jan 2008 and I was told it was the catalytic converter, so I had new one installed and it then passed (don't recall the NO emission figures). Then in Jan 2010 (18K miles later), it failed again with high NO emissions of 947ppm vs 785ppm max at 15mph/1786 RPM despite the new catalytic converter. This time, they said it was the Oxygen Sensor, and so they replaced it and the car then barely passed at 775ppm NO (1747 RPM). Now I must have it tested again (this month) and am concerned it may not pass again. I am out of work now and cannot afford more major smog repairs or a new car. (Yes, I am aware of the state's $500 repair help.) Any advice on what might be wrong, or how I can get it to squeak by and pass this time? Someone told me to drive the car for a while before having it smogged??? What about testing on a full or low tank of gas or getting an oil change in advance? Can anything like this really knock off a few NO PPM? Any advice would be greatly appreciated. Thanks!
Answer:

The advice of replacing the CAT in order to pass a smog check is not recommend, in fact is illegal. A smog check repair station needs to conduct a thorough diagnosis prior to condemning a Catalytic Converter. It appears in this case the CAT was replaced unnecessarily in 2008, and the real culprit was the air fuel mixture. Something is causing your BMW to run "hot", creating high NOx. It could be a lean fuel condition, it could be high compression due to carbon build-up on the piston heads; pretty much any condition which will cause your engine to run hotter than normal. We recommend starting with a wet/dry compression check, followed by a fuel feedback test to ensure your BMW is in proper fuel control, and then a vacuum leaks check, which may be presenting unmetered air to the intake system making the air/fuel mixture too lean. Above all this however, make sure you are using high octane fuel. Higher octane fuels burn much cooler than low octane. posted by SmogTips Support

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