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The EGR valve may not be functioning correctly allowing too much oxygen to enter into the combustion process. If someone has rebuilt the engine the compression could be at a high enough level that increased levels of oxides of Nitrogen could be produced. Note that high compression and faulty EGR valves that allow too much oxygen both cause the combustion temperature to run higher than designed. High compustion temperatures are the ultimate cause of elevated levels of oxides of Nitrogen. *Engine could also be running too hot and casusing incresed combustion temperatures causing increased oxides of nitrogen

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โˆ™ 2006-05-27 12:36:11
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Q: What can be the cause of high NO levels in an emission test?
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