Why NOx too high at 15 mph California smog test?

[davidk]

My 93 (253K miles, I've had it since 1999 - 77K) has a history of failing the 15 mph California smog test (smog test required every two years). The highest NOx allowed is, say 560. When it failed the 15 mph test six years ago (putting my X in gross polluter category), the NOx was 700. New catalytic converters put the NOx at 15 mph at 0. Two years later, it failed 15 mph test again with NOx at 690. New catalytic converters put NOx at 0 (new California regulations mandated better catalytic converters). Two years later it passed 15 mph test (barely) at NOx 490. A couple weeks ago (two years later), it failed 15 mph test at NOx 670. New catalytic converters put NOx at 0.

No check engine lights. Starts and runs great, gets good gas mileage on highway, not so great around town.

I periodically clean the MAF, put in new plugs and wires a year ago, and I'm tired of replacing the #$%@ catalytic converters every other smog check, although it is a LOT less money then it used to be!

Thanks for any suggestions!

David

 

[FR-425]

ww.easterncatalytic.com/education/tech-tips/look-upstream-for-the-culprit-in-a-converter-failure/

 

[natenkiki2004]

NOx is usually formed when the engine runs real hot. Might be your EGR system isn't working properly? It should kick in at 15mph since it doesn't at idle or WOT.

 

[FirstExplorer]

I was informed by a very good auto technician that the cats made today are pretty crappy when compared to the OEM cat your Ford had originally.
The rare earth materials needed to make a good cat are so expensive that the new ones are deficient in these materials and burn out quickly.
Now I passed NOX one year after failing by dumping a half gallon of Everclear into the tank. Drove 20 miles and retested and passed. BTW, while on the dyno, the exhaust smelled like hot martinis. But, and that's a BIG but, I think that alcohol fried the cat. Fried cat, BTW, is very popular in Korea.

 

[davidk]

Thanks for the link to the cat info, the suggestion to check EGR system, AND the Everclear "hack!"
I was told by a smog tech that before recent California regulations, most of the "California" cats were known in the industry as "20-minute cats" since that was pretty much how long they functioned - just long enough to pass smog. Now the general public can buy California cats (we couldn't, before) that are better quality, even at places like Autozone.

 

[Centaurious]

It could also be related to additives in your fuel, try changing brands.

 

[malohnes]

"Now I passed NOX one year after failing by dumping a half gallon of Everclear into the tank. Drove 20 miles and retested and passed. BTW, while on the dyno, the exhaust smelled like hot martinis. But, and that's a BIG but, I think that alcohol fried the cat. Fried cat, BTW, is very popular in Korea."

Haha - fried cat...good one.