NOx has nothing to do with engine operating temperature as expressed by coolant temperture. NOx is created in the combustion flame front which runs about 4000-4500 degrees F.

For every engine speed and load there is an ideal ignition timing that will create maximum thermal efficiency, which means lowest fuel consumption. This, unfortuately is also the timing that creates maximum flame temperature, which also produces maximum NOx.

OEMs have to walk a fine line between NOx standards and fuel efficiency, so depending on the model, igntion timing may already be slightly retarded from the ideal amount that would maximize efficiency. Retarding from this point further reduces peak combustion temperture and increases EGT. The former will reduce NOx and the later will usually reduce HC and CO, so retarding the timing is a quick way to turn a marginal failure into a pass. Sometimes only a drop of a couple of hundred degrees in peak flame temperature will reduce NOx significantly because there is a "knee" in the reaction rate curve at about 4000 F. Above this NOx can increase dramatically. Likewise, below the knee in the curve NOx is relatively low.

Modern cars are becoming more "tamper proof", and this includes eliminating the option to vary initial timing or the rest of the timing map as established for engine speed and load. Such is the case with many Mercs. For example, on the M103 engine we can't adjust the initial timing, but changing a plug-in resistor will slow the rate of timing advance with engine speed and there may also be a chance that vacuum advance can be eliminated. Either of these will reduce the timing that the engine sees during the emission test and should reduce emissions.

Since I'm not familiar with the igntion systems on BMWs I can't give you any guidance on how to reduce your timing, but I would imagine that there must be some BMW discussion boards with someone sufficiently knowledgeable about the BMW ignition system to have figured a way.

That's the best I can do on this one.

Despite the fact that field emission testing has been in place in many parts of the country for over 20 years, there seem to be few techs who understand how emissions are formed, how they are controlled, how an engine can be "adjusted" to correct a marginal failure, or even read the test data on an emission report and use it as the beginning point in a diagnosis. Instead, they mostly want to sell you "tuneups" and "fuel injection service" or other services that are profitable for the shop, but often don't get at the root cause of the emission test failure. I'm glad that the CA test reports O2 content, because it is an important diagnostic tool that can get one started in the right direction toward solving an emission problem.

Duke