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Forum > Failed Smog Test > Question

1991 volvo 780 turbo 2.3 L, nox emissions were really high other emissions were good

what is causing the nox emissions to be so high? failed at 15mph and 25 mph for the nox emissions only other emissions passed easily.I am thinking it could be the cat or egr related the only thing ai don't think there is a egr valve on this turbo engine, thanks if you can help.
Answer:

Your right about your vehicle not having an EGR valve. If it did, the EGR valve would help the reduction of NOx. Since it does not, the reduction of NOx is mainly up to the Catalytic Converter, and the fuel feedback system. If your Volvo is running lean it will create high NOx, so it's important to ensure there are no vacuum leaks or defective fuel control. Engine cooling problems may cause high NOx also. Remember NOx (Nitrous Oxides) is created engine temperatures reach over 2500F. For this reason you want to make sure your vehicle's temperature gauge is always normal and that your cooling system is working properly. High compression can cause high NOx also. Carbon build-up within your engine's combustion chambers will cause increased engine temperature. Keep in mind this problem is usually seen in vehicles with over 200,000 miles. The solution to this problem is called an Engine Decarbonizing. Decarbonizing your engine will remove carbon of your engine's piston heads and valves. This will lower compression and lower NOx. posted by SmogTips Support

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