Toyota Repair: 90 Corolla Failed Ca. Smog, combustion temperatures, exhaust gas recirculation

Question
Hi Ted,

 I have a 90 Corolla, 1.6L Auto Trans. 293,000 mi. It has failed the smog test twice for high NO(PPM). The Functional and Visual passed both times. The first time emission failed

15 mph RPM 1985  %Co2 MEAS 14.4 %02 MEAS 0.2
           MAX       AVE      MEAS
HC (PPM)    124        31       97
CO (%)      0.78      0.10      0.12
NO (PPM)    815        237      1288

25 mph RPM 1963  %Co2 MEAS 14.6 %02 MEAS 0.1
           MAX       AVE      MEAS
HC (PPM)    98        20       39
CO (%)      0.66      0.09     0.05
NO (PPM)    754       199      1083

I used the Guaranteed To Pass Smog fuel additive had it retested ( little better) but still failed.

15 mph RPM 1934  %Co2 MEAS 14.27 %02 MEAS 0.86
           MAX       AVE      MEAS
HC (PPM)    124        31       48
CO (%)      0.78      0.10      0.21
NO (PPM)    815        237      1111

25 mph RPM 2335  %Co2 MEAS 14.80  %02 MEAS 0.18
           MAX       AVE      MEAS
HC (PPM)    98        20       31
CO (%)      0.66      0.09     0.12
NO (PPM)    754       199      399

My funds are limited so hopefully I can get it to pass without taking it to a repair shop. Any ideas or suggestions you can give me would be great.

Thank You for your time and help.  

Answer
Higher than normal NOx readings are almost always due to a malfunctioning exhaust gas recirculation (EGR) system, if it passed the functional egr test then I have trouble understanding why the readings are too high, another cause may be  that the engine timing is too far advanced but this should also have been noted on the functional test, the only other things I can think of is that the  combustion temperatures are too high due to the engine running hotter than normal, could this be the case? Or that the catalityc converter is having a problem.