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1990 Chevy Corvette failed smog test for high NOx for the first time since new!

I own a 1990 Chevy Corvette, not ZR-1. My Vette failed the smog check for the first time since new. Everything passed except the NO(PPM). At 15mph it MEAS was 2217 (well above max). At 25mph MEAS was 1498 (well above max as well). Any idea on what the problem may be? It has always been serviced. In fact, just had a tune-up before the smog inspection.
Answer:

We have to know what amount of CO your Corvette produce. This will tell us whether the cause of the high NOx is due to a lean fuel condition or not. If CO is very low (0.00 to 0.05) your Chevy Corvette may have a fuel management problem; maybe a bad oxygen sensor/s, mass air flow meter, pressure sensor, or possibly clogged fuel injectors. These components will have to inspected. If CO looked good (near average specs) you want to concentrate on the EGR valve and EGR system. Your Corvette's EGR valve is responsible for lowering NOx by re-circulating a small amount of exhaust gas (inert gas) back into the combustion chambers. Nitrous Oxides (No) emissions are formed when combustion chamber temperatures rise above 2500F. Presenting an inert gas keeps the chambers cool and reduces NOx. Other problems that could cause your Corvette to produce high NOx could be bad engine cooling, lean fuel condition (though a lean fuel condition would be suspected if CO levels were super low 0.00 to 0.05, as I mentioned) and high mileage which could mean there is carbon build up on the valves and piston heads of your Corvette's engine increase compression and engine combustion temperatures. posted by SmogTips Support

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