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Most vehicles built in the '90's were built with EGR valves, whether visible or not. Look on your emissions sticker for the moniker EGR. It will appear with stuff like o2, catalyst, etc. This will assure you that the engine either has EGR or not. Though possible, it is not typical for this era engine to not have EGR. If it does not, things like excessively lean mixtures, or over-advanced timing can create excessive NOx. It is also possible for an incorrectly installed timing chain to cause this as well. Essentially, excessive combustion temperatures cause high NOx emissions. Let me know ... Ray
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