NOX is a by-product of too high of combustion temp. A very common cause of high temp is effective compression ratio. Carbon! These cars form lots of carbon due to oil consumption through the valve guide seals.

Once one has verified the timing control and EGR activity then a good decarbonizing is in order.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician