Well, the old '91 300SE finally failed the Dallas emissions test, HC and NOx at low speed. It passed at the high speed test.
Below are the gory details.
If bored, please skip to the last sentance, that is my big question, I guess.

25MPH 15MPH

HC 108 97 112 173 (fail)
CO% 0.60 0.16 0.62 0.31
CO2% 14.8 14.4
O2% 0.2 0.5
Nox 751 453 829 1171 (fail)
Dilution >6 15.0 >6 14.7

Last year she passed with readings approx 1/2 the limit, and not a lot has changed in the way she runs.
So, I think I'll try the following?

1. Clean the EGR line. (been awhile).
2. Double-check for vacuum leaks. But I just replaced the IAC hoses awhile back, all hose connections are tight, and the injector seals are pretty new - vacuum at idle is 16".
3. Top-end cleaner (Seafoam).
4. Fill up with Super Chevron (been running midgrade Chevron lately).
5. Change the oil (but it's pretty new).
6. Change air filter. Plugs were brand new when I failed today.
7. Recheck the Lamda duty cycle, but it was at 48-52% and fluctuating well just last week.
8. O2 sensor is original, but seems to be working ok, per the duty cycle monitoring. David monitored the sensor on his cool equipment last month, seemed ok then. At 17 years maybe I just ought to change it?
9. Drive the crap out of it before taking it in.
10. ??? anything else?

Would the Lamda setting affect these emissions? Mine is dead-center at 48-52% and moving well, per my Sears duty-cycle meter. Seems to me, if the Lamda were lowered or raised a bit, the sensors would take over and move the mix to Stoi anyway. But would changing the duty cycle actually change the closed-loop mixture, and the various emissions levels? if so, should I decrease or increase Lamda percent to improve the HC and NOx readings?

Thanks,
DG