I had failed my first smog test last month with my 89 745T. (http://brickboard.com/RWD/index.htm?id=776778)
I got a new air filter, had the AMM adjusted, checked timing (OK), and all hoses. I found a the long vacuum hose that goes from the turbo to an intake manifold fitting the nipple very loose, so I replaced it. I also got a new catalytic converter made by DEC for a good price. My parts guy spoke highly of them, and a BMW online parts house recommended them 2nd only to the OEM BMW cat. Despite this, I was severly disappointed with the fit. Like other aftermarket cats, the o.d. of the rear pipe was less than on the original cat I was replacing, making it difficult to get a good seal. Also the angle of the pipe coming out of the cat was off by about 5 degrees. I ended up heating and bending the pipe slightly and expanding the end of it to increase its' diameter slighty with a torch. This gave me a good relatively leak-free seal.
Went for a re-test (at no charge) and passed. At 15mph HC measured 96ppm(116 allowed), CO .43%(.74% allowed), and NOx 1023ppm(1044 allowed). At 25mph it produced 23ppm HC(91 allowed), .04%CO(.62% allowed), and 173ppm NOx(904 allowed). Notice that at 25mph it passed by a big margin, so much so that it is even under the average for all passing cars on CO(.07%) and NOx(194ppm). But at 15mph it isn't under the max by much on HC and NOx, which makes me wonder what will happen it 2 years when I have to retest.
I wonder what I could have done to lower the HC and NOx at 15mph. Perhaps a better cat that is much hotter at 15mph. Timing was at 12, so would going down to 10 been better or worse? And also maybe putting the 357 injectors that the car came with instead of the slightly higher flow 804's. The car as tuned to pass (was too rich before) hasn't produced any better gas mileage, and seemed to have a bit more power before, but I could be wrong.
Thanks for all the good advice on my first post that helped me pass.