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Failed Smog Test

BMW 525I Failed Smog Check Twice

My 1993 BMW 525i failed it's smog check twice for high HC both times. First test results: 15mph HC was 116. Everything else was below the maximum. After the first failure, I changed the spark plugs, air filter, and did an oil change. But on the second test, my results were worse. 15mph HC was 220 (max 83). NO was 1245 (max 689). And 25mph HC was 79 (max 48). My CO was 0.38 for low 15mph and 0.21 for high speed. Any tips on what I can do? Will a new CAT help? Also, I notice that the spark plugs were drowning in oil. I am in the process of changing the valve gaskets to stop this from happening. Did that cause the smog failure?


Let's start with the last thing you mentioned first... The leaking valve cover gasket is a big problem. It can absolutely be the cause of your BMW's smog check failure. We highly recommend replacing the gasket(s) and driving your BMW for at least 500 miles (and while using engine cleaning additives) in order to burn off the carbon buildup which has accumulated on the valves and engine head. Increased engine compression is one of the leading faults responsible for high NOx. Not to mention, if you had oil seeping in to the combustion chamber during the smog check, air to fuel ratio would have been disrupted... further leading to increased emissions. Once the gasket problems have been resolved and the engine decarbonized as best as possible, you should have the vehicle retested. From there you can continue the diagnosis in the unfortunate event your car still fails.

posted by SmogTips Support 11-30-2017 10:10 AM

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