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1982 Cadillac Seville 4.1HT fuel injected

My Cadillac Seville has failed the smog test 3 times each time I get closer to fixing it but this last time it failed for high NOX gases. 15mph - (max)742 (meas)919 25mph (max)623 (meas)1053. It passes every where else. I've replaced the thermo vacuum switch and the egr solenoid. Why is the NOX so high and how can I fix it.
Answer:

We're going have to look at the CO results as well as the NO results. It is possible the high NO is a result of a lean fuel condition. If in fact the EGR system is working; the EGR valve opens as required, and when it does open, it sends exhaust gases back to the intake manifold then you have to ensure there is no lean fuel condition. Try opening the valve manually during idle. The idle speed should drop almost to the point of stalling the engine. This indicates the EGR valve is working. You then have to make sure the solenoid is receiving power during off idle when your Seville is being driven. If that too is working, then you'll need to make sure the oxygen sensor is sending a signal to the computer and the computer is adjusting fuel flow. posted by SmogTips Support

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