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Forum > Failed Smog Test > Question

1984 Chevy 1/2 Ton Truck failed NOx (ppm), why?

I passed everything except NOx (ppm) at 25 mph. The Max allowed was 760. My Chevy Pickup meas 1131. What do I repair to pass? Is it a sensor? And where would the problem be in my truck?
Answer:

If your Chevy truck's CO percent was normal (between .1 and .5) then we'd recommend you look into the EGR valve (exhaust gas recirculation system). The EGR valve is responsible for lowering NOx by re-circulating a small amount of exhaust gas (inert gas) back into the combustion chambers while the engine is running. Nitrous Oxides (No) emissions are formed when combustion chamber temperatures rise above 2500F. Presenting an inert gas keeps the chambers cool and reduces NOx. The EGR valve on your Chevy pickup should be inspected to ensure it's working properly. The EGR valve should be able to open using manifold vacuum. When the valve is opened at idle you should notice a dramatic drop in RPMs. The EGR system over time collects carbon. In time it may become clogged and stuck open or closed from carbon deposits. Other problems that could cause high NOx if the EGR system is working properly 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) and high mileage which could mean there is carbon build up on the valves and piston heads increase compression and engine combustion temperatures. posted by SmogTips Support

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