Ok, '91 Miata, 171,000 miles, fails NOx on emissions.

Already put on a new O2 sensor and a used cat (was on a gutted one).

Blows 1400 ppm, needs to be under 1100 ppm.

What are some fairly easy things to try to keep this sled on the road?

To be on topic, we do our HPDEs in this car.

Thanks guys,

Bowie, who really can't afford this crap.

 

Exhaust wrap around the header and cat to help generate heat faster.
Or a NEW cat.

High flows go for 50 bux on Ebay.

 

It could be running lean, is there any way you can add a touch of fuel?

High NO generally is caused by lean fuel/high combustion chamber temps. I had the problem with my Si, added a little fuel with a VAFC, ppm went from 1220 to 190

Brian

 

Well, if the car is burning a stoic A/F mixture, the problem is most likely carbon buildup in your EGR system. It's possible your cat could need replacing, but normally you'll be high on CO as well. I say this as I did a lot of research after my beater failed emissions on the NOx count 10 days ago.

 

less ignition advance. had a buddy that had the same prob. with his SM and I'm pretty sure the fix was setting timing back to the factory spec.

 

Cool, thanks to all.

Here is the plan: Set the timing back to 10 degrees (at 14 now), replace PCV, and up the fuel octane from 89 to 93 for the test.

That oughta bring the combustion temps down.

Hopefully it will pass this time, this is getting old.

Bowie