I keep failing due to high NOx. Before the first test I changed the oil and topped off with fresh gas in a truck that I don't drive very much. Test results:
10/26/11 15mph 25mph
%CO2 13.9 13.5
%O2 1.6 2.3
ppm HC 150 28
%CO 0.25 0.01
ppm NO 1502 1233

I installed a new O2 sensor and installed new plugs. Confirmed that EGR was functioning properly. Test results:
10/27/11 15mph 25mph
%CO2 12.1 11.4
%O2 4.0 5.0
ppm HC 82 31
%CO 0.00 0.00
ppm NO 2403 2098

I adjusted the valve clearances and replaced cap and rotor. None of the valves were off by more than 0.001" and the cap and rotor had fewer than 5K miles on them. Test results:
11/11/11 15mph 25mph
%CO2 13.7 14.0
%O2 0.2 1.3
ppm HC 247 55
%CO 1.80 0.01
ppm NO 1359 1382
Installed a new MAGNAFLOW direct-fit catalytic convertor. Test results:
1/11/12 15mph 25mph
%CO2 12.6 11.0
%O2 3.4 5.6
ppm HC 19 15
%CO 0.00 0.00
ppm NO 1951 1757

Replaced functioning EGR valve and vacuum modulator. Test results:
1/12/12 15mph 25mph
%CO2 14.0 12.4
%O2 1.6 3.5
ppm HC 14 12
%CO 0.01 0.00
ppm NO 1496 1322

During a subsequent "free" test at my local smog shop, the technician temporarily disabled the EGR during the 25mph test. The NOx jumped from about 1500ppm to about 3200ppm. The EGR seems to be doing its part.
Is it possible that the brand new MAGNAFLOW cat is defective?
Any other suggestions will be appreciated.

 

Found this link that gets into what can cause that. Sounds like hot combustion chambers
http://www.nichols.nu/tip766.htm

 

With an 02 sensor change i went from a
%co2-13.0
13.0
%02 - 2.7
2.6
HCrpm -132
128
CO% - .60
.61
NOX - 2808
2869
Holy crapola
then the sensor change.........

NOX -6
2

 

What brand O2 sensor did you install?

 

It was a Denso.108.00$. My log book showed the old one installed in 98. 14 years was enough. I fully expected the shop to find other things wrong to correct bad smog. so me putting in the new on was a good move on my part. I got it at Autozone or Napa auto parts, can;t remember.

 

The footnote on your posts suggest that your truck is an '88 22RE. In that case the O2 sensor controls your fuel injection so I can understand how it might have a dramatic effect. My old '82 22R is carbureted and I can't understand how the O2 sensor can effect the fuel/air mixture. All the same, I genuinely appreciate your help and if my decarbonization effort does'nt improve my NOx results I may replace my new Bosch O2 sensor with a Denso unit. Thanks again.

 

NP and good luck....I hate smog bs.....

 

I re-tested today after decarbonizing the cylinders. NOx dropped from 1300s to about 1050 ppm. Still not good enough. NOx at idle was 120ppm. Technician said it should be 0. Technician thinks my carb is set too lean. There is no adjustment except to drill out the jets. Does anyone have any ideas?

 

SUCCESS! The problem turned out to be that the fuel bowl float was set way too low. I may have done that when I rebuilt the carburetor. I did not change the needle or seat because it's a pain to re-set the float, but I may have bent the float-adjusting tab by accident. I could still achieve 75mph on the freeway but the low bowl level apparently caused a low enough pressure to the venturi to create the lean condition that caused the high NOx.

 

NIce! Thanks for posting up the fix for future reference.