1998 Chevy S10 failed smog check for high NOx.

The most common reason for failing the NOx portion of the emissions test is a malfunctioning EGR system. The EGR valve recirculates exhaust gas back into intake manifold. This cools the combustion chamber temperature down and reduces NO emissions. Nitrous Oxides (NO) emissions are formed when engine chamber temperatures rise above 2500* F. Keeping the chamber cool reduces NOx emission. You should inspect your Chevy S10's EGR valve for proper operation. A working valve should be able to open its passage using manifold vacuum. Over time the EGR system will is develop carbon build-up and might become clogged or stuck closed. The testing process should be simple. To verify whether the EGR system is working start your truck and let it run at idle. Disconnect the EGR valve's vacuum line and apply direct vacuum (using a vacuum hand pump) in order to open the valve. When the valve opens; if the passages are clean and open, you should notice a large RPM decrease. If the passages are plugged up, you will not see any RPM diffrence. If there is a RPM decease, then the passages are free, and the next step is to inspect the vacuum signal which is supposed to open the EGR valve. It may be possible your vehicle is not opening the EGR valve at the right time, or at all. If there is no RPM decrease, the passages may be plugged up. You will need to disable the EGR valve and clean all passages within the valve and within your engine's intake and exhaust manifold. If the EGR system checks out ok. We'll have to find out if your Chevy S-10 is in proper fuel control. The diagnosis should begin a “fuel feedback” test, ensuring the oxygen sensors are working properly, and the ECU is sending the fuel injectors the correct information in terms of increasing or decreasing fuel delivery. At the end of the feedback test it will be know whether your Chevy S-10 is suffering from a Lean fuel mixture problem (which is causing high NOx). Other NOx faults to concider are: bad engine cooling, carbon buildup in the cylinders due to high mileage, and intake vaccum leaks.