Car:
1989 Model 240 Sedan 5 speed trans 209,000 miles odometer

Problem:
I have been working on my car for a number of years trying to track down the cause of high NOX test readings. Six years ago the numbers were in the 200ppm range and they have been drifting higher when the inspection measurements are done every two years. Last week I just squeaked by with a number just 10ppm under the limit of 1300ppm.

Changes/repairs made to solve the problem were/are:
1-New catalytic converter and new exhaust pipe from the manifold to the flange inlet of the converter (new gaskets between the parts)
2-New oxygen sensor
3-Compression check shows 145psi on all cylinders
4-Valve clearance check shows all valves to be at mid-range of clearance
5-Treated gasoline with Chevron fuel additive for injector cleaning
6- New spark plugs, spark plug wires, distributor cap and rotor
7- Oil changes every 3000 miles since mile zero with filter change
8- Engine temperature sensor replacement prompted by fuel computer fault indicator and subsequent LED code.
9- Checked and cleaned all fuel injector engine and chassis ground contacts points.

Other observations:

A - Fuel and Ignition computers show no faults through use of the blinking LED message system.
B - Car performs very well with good acceleration and 30 mpg highway/27 mpg city driving
C- Over the years I have also replaced the fuel computer, air mass flow measurement sensor and the PVC system/flame trap several times.

I would appreciate suggestions on other routes to investigate that I may have missed based on other owners experience.