Coolant temperature has little to do with NOx. NOx is produced in the 4500 degree flame front, and there is essentially no difference in flame front temperature (for a given set of operating conditions) within the normal range of coolant temperatures. The higher the flame front temperature, the greater the NOx produced, and there are strategies to decrease engine-out NOx to a level that will be acceptable at the tailpipe after further reduction reaction in the catalyst. (I use the word "reduction" in the chemistry context. It is the opposite of oxidation.)

Higher coolant temperature mean hotter combustion chamber boundaries, which reduces flame quenching at the boundaries, so engine out HC is usually lower with high coolant temperatures.

Spark timing has a big impact on both engine out NOx and tailpipe HC emissions. For every operating condition (speed and load), there is a timing value that both maximizes torque and minimizes fuel consumption. This "ideal" timing will also produce the greatest flame front temperature and NOx, especially under "lean" i.e. stoiciometric condtions, so timing under many operating conditions is decreased below the ideal to decrease engine out NOx. Retarded timing also increases EGT, which helps keep the converter hot enough to generate good oxidation and reduction efficiency to decrease engine-out emissions to acceptable tailpipe values.

The "ideal" timing is that which produces the greatest thermal efficiency, and greater themal efficiency requires adding heat at higher temperature, but if NOx is too high, timing must be retarded. This essentially reduces the fuel's contribution to useful energy at the crankshaft and throws more energy out the exhaust, which, of course, increases fuel consumption.

The other strategy to control engine-out NOx is EGR. By diluting the fresh charge with an inert gas, peak flame front temperature is reduced.

A simple stategy that car owners can use to reduce both NOx and HC on a car that may have marginal emission test performance is to retard initial timing. Retarding the timing reduces peak flame front temperature, which reduces engine out NOx. Also, the higher EGT from retarded timing will keep the converter hotter, which will increase its oxidation and reduction efficiency.

Unfortunately this is not an option on many Mercedes models because the intial timing is not adjustable - like my M103 engine.

Duke