Barely Failed CA Smog - Nox @ 25mph

Hi all,

We just moved to CA from Washington. I've been driving my 1998 4Runner (RWD, Auto, 248k) daily for years. In January, I had the original OEM cat stolen in Seattle. I've driven the vehicle with a straight pipe since, including our move down to CA.

To attempt to pass CA smog, I had an aftermarket Magnaflow CARB compliant cat installed this week, and have about 300 miles on it.

I took it into smog today, and 5 of 6 CA tests passed, except for my tailpipe NOx reading at the higher, 25mph simulated speed. HC, CO2, and NOx at 15mph all pass comfortably. I'm barely failing at 25mph - off by a total of 33 PPM, or roughly 4% of the allowed level.

My understanding is that the higher NOx values are usually the by product of a combustion chamber running hot. Any initial diagnostic ideas to try?

The fact that HC and CO2 levels look fantastic tell me the new aftermarket cat seems to be working. But I'm not sure why the the NOx reads comfortably healthy at 15mph, but fails at 25mph?

I have the 5VZ-FE, and there's no EGR valve, which was a common issue when smogging the 3.0L models.

Ideas:

- Driving with a straight pipe generated more carbon build up, so I'm running hotter?

- Some other pressure/vac element isn't working (would PCV value effect this?)

- New cat is still not "broken in"?

- New cat simply can't keep up with NOx removal at higher loads?

Ideas welcome. I'd hate to have to rehome this over 33PPM...

Thanks!