High NoX levels are usually related to a lean mixture which burns hot. Look for vacume leaks...and try a can of BG44K wich buy the time you are through the tank should clean out any crud that has developed on the injectors...

Check your fuel filters also.....a clogged filter will drop Ijector pressure and lean out the mixture too.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche