Toyota Repair: Emissions NOx corolla 92, spark plug wires, nox emissions

Question
92 Corolla 4A-FE Engine, Auto Trans, 190K miles

Hello and thank you for providing your services!

Here is the question: My car failed NOx on Emissions (in Maryland). Here are the readings:
HC: 0.578 GPM (State 1.0 GPM) Pass
CO: 5.324 GPM (State 20 GPM) Pass
NOx:2.557 GPM (State 2.2 GPM) FAIL!

Before going for emissions test, I have changed engine oil, and did a major tune-up (replaced air filter, PCV valve, rotor, distributor cap, spark plug wires, 3 spark plugs(1 is seized/welded), radiator drain and re-fill). I also used a fuel system cleaner, slick 50.

After the test, I have changed the fuel filter (as this might be causing a lean mixture responsible for NOx), tightened the clamps on air intake duck (between air filter and throttle body), used a throttle body cleaner from NAPA. Now running another fuel carbon deposit cleaner by LUCAS.

Local Mechanic recommend changing catalytic converter, which is expensive. I am looking for other small improvements to reduce NOx emissions.
I do not know if the catalytic converter was changed. It looks old but how old, no way to find it.

I also have the results (graphs) from Emissions office that show levels of NOx (and ofcourse of CO,HC) during the test time and the speed. I can explain if anything out of it can help. Only particular thing I can observe is the NOx values are higher during acceleration, while CO and HC do not do this.

One of the reason for high NOx is advance ignition timing. I asked a shop to check the ignition timing and the idle speed and I was told that it is set by computer(ECM) and can NOT be fixed manually. Is this true about this years corolla?

From your experience, do you know anything in particular that causes high NOx on these particular model year corollas?

Thanks for your help!


Answer
The most common cause for increased NOx emissions is higher than normal combustion temperatures, this can be caused by a malfunctioning EGR system, engine overheating, compression too high because of carbon build up, misadjusted timing.
The timing is adjustable by rotating the distributor, it should be set at 10 degrees BTDC with a jumper wire in the diagnostic connector between terminals E1 and TE1, the reading is very close to the limit so I doubt if the catalityc converter is the problem.
You will probably get to within the limit by making sure the timing is set correctly so find another shop that can adjust the timing.