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I had never heard this suggested as a benefit of adding alcohol to fuel, but it makes sense that adding an oxygenate and lowering the energy density of your fuel might lower the NOx on a '91. It would certainly help with CO.
Your tank must have been REALLY low for a (I presume pint) bottle of alcohol to make a significant difference.
Also, the owners manual on a '91 warns that the fuel system is not compatible with alcohol, so you would not want to make a habit of this.
I agree with most responders that the CAT is most likely going to end up being the solution. The key is the oxygen level in the exhaust. If you have high oxygen level in the exhaust, then the problem is an air leak or a control system failure. A good CAT cannot reduce NOx effectively if there is too much oxygen present. Looking at my old emissions tests for my '91, you are looking for less than 0.5% O2 at no-load fast idle, or less than 0.1% O2 on the loaded test. You can tolerate up to 1% O2 at no-load idle where NOx is not a problem. The problem is that the O2 meter is the most failure-prone meter in the shop's test unit, so you cannot always trust the readings you get from the smog test report. I have one report that says that Oxygen was 0.00% at all speeds. Uh huh.
Another possible culprit for NOx is excessive carbon buildup in the cylinders.
Using premium rather than regular might drop a few ppm NOx if you are right on the borderline, but it won't get this car where it needs to go.
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