Barely Passed Smog

The old monster passed again. All flying colors with wide margins, except for the 15mpg NO -- was right at the 550ppm (I'm guessing from memory) maximum. 25mpg was well under that limit.
Outside of major replacements, what would bring NO down?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 15 Aug 2015 10:42:24 -0700, "Snuffy \"Hub Cap\" McKinney"

NOX goes up as combustion temp goes up. If your EGR is not working your Nox goes up. If the wrong thermostat (too hot) is installed, NOX goes up. If the rad is bad, raising operating temps, the NOX goes up. If timing is too far advanced, the NOX goes up. If the engine is running too lean, NOX goes up, and of course, if the catalytic converter reduction bed is failing, NOX out the pipe will also be high.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

except for the 15mpg NO -- was right at the 550ppm (I'm guessing from memory) maximum. 25mpg was well under that limit.

Thanks. What would make NOX run so much higher at 15mph and not 25mph?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 18 Aug 2015 10:36:02 -0700, "Snuffy \"Hub Cap\" McKinney"

My first thought would be a restricted EGR system. Take off the EGR valve and tubing and clean out all the carbon from the system.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.