Well, it is that time of year for me here in Georgia, the annual emissions test. I have a 1985 500SEC, US car, all original, 210,000 miles, great shape, 22+mpg, less than 1 quart in 5000miles, all systems fully functional, NEW reduction catalyst and NEW converter catalyst.

Problem, Georgia tests this car for all emissions including NOx. This car has NO EGR system. No not removed, never installed, no valve, no port, no bleed tube, no EGR, simple. It WILL NOT PASS the NOx part of the test. Temperatures are good, even the emissions repair stations say there is no problem with the car, it just will not meet the spec.

We tested the car on the dyno in 2nd gear so the cats would be hot enough to function, it helped greately but still missed the spec by 50%. All adjustment are at factory specs, all components functions as built. Last year i riched up the fuel so bad that we drounded the NOx out, it passed the emissions test, but ran so poorly that it failed to get me home(1.5 miles!)

HELP! I am ready to try most anything at this point.

First thing that comes to mind is to suspect the oxygen sensor in the exhaust. Not sure you have one in that car - but thought I'd throw in the idea.

Someone might comment on generally what causes high NOx emissions. I think its fuel mixture that's burning too hot, causing the nitrogen in the air to burn also.

Hmmm. You have the right spark plugs in the car with the right heat range?

I always tell people to make sure they are running premium fuel for a few tanks before the test - you probably already are doing that.

Just grasping at ideas here........

Ken300D

In general,high NOx is caused by high internal engine temps, or a damaged three way cat.
Also could be:
initial timing,
spark advance system,
carbon deposits in the combustion chamber, a possibility for this car with so many miles on it.
or a leaking or malfunctioning air pump.

good luck, Oh,
PS. you might want to check the various sensors that control engine temp.