The higher the coolant temperature the lower the engine out HC. Coolant temperature has little/no effect on NOx or CO.

As the 4500 degree flame front approaches the combustion chamber boundaries it is quenched because the surfaces are cool relative to the flame front, but the higher the boundary temperature the less quenching occurs.

CO formation is primarily dependent on fuel-air ratio, and anything richer than stoichiometric dramatically increases CO in proportion to the excess fuel.

NOx is formed in the 4500 degree flame front. Cooling the peak flame front temperature by either retarding the timing or diluting with fresh charge with inert exhaust gas lowers peak flame front temperature and NOx formation.

Duke