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BMW 325i Why Did my HC and NO Jump Up So Dramatically in Two Days?

I own a 1995 BMW 325i. I took my BMW in to get a smog check and they failed it because the gas cap was bad and the oil was dirty. Both supposedly didn't hold up to their standards and I needed to put back on the air box that was originally on it . But my HC was at max at 85 and my NO was at 60 and max was 695. I change the gas cap and air cleaner and took it back two days later. Now they tell me that everything was fine except now my BMW failed because my HC is 154 and my NO is 186. How is this possible?

Answer:

There can be a few possibilities. You first should make sure the air cleaner assembly you installed does not have any vacuum leaks. In other words, if there were any vacuum hoses connected to the previous air box they must be properly connected to the new air box. Even a slight intake vacuum leak will cause high HC. Plus make sure nothing was disturbed while replacing the original air cleaner. The second condition is simply having your BMW's catalytic converter not hot enough. A vehicle will never produce identical emission results during the emissions test. The temperature of the CAT has a lot to do with HC reduction. The hotter your BMW's catalytic converter during the smog test, the lower the HC; just be careful of NO. NO increases with CAT temperature. It's a fine balance. At any rate, HC should be well below 60 ppm during the smog test. With HC amounts such as this, most definitely an emissions problem exists which will need to be diagnosed. Read the following for more HC faults. What Causes High HC? posted by SmogTips Support

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