Is it harder for the pre-'90 cars to pass smog since they don't have EGR or an air pump? Seems like that would be the case.....

MI rocks, no smog tests here!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

NOx forms when cylinder temps reaches 2500F and higher which indicates a lean condition. A lean mixture will produce oxides of nitrogen since carbon monoxide and hyrdocarbons are down.

I would have the lamda adjusted and the oxygen sensor tested out. Then I would make sure the fuel delievery system is delivering a good amount of fuel.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

I was at an auto parts store and chuckled but was curious when I saw not one but two different additives claiming to "guarantee" smog pass...

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'88 300TE, 175k, black RENNTech 3.6L Recaro C's AMG 1 SOLD
'92 500E, 110k, Spruce green, stock SOLD
'94 E320 Cabriolet, 130k, E500 wheels, Emerald green SOLD
'94 E320 Cabriolet, 110k, black, stock, SOLD
'88 300TE, 229k, dark grey, SOLD
'90 300CE, 212k, white, new paint, SOLD
'91 300E, 209k, white, rebuilt head SOLD
'74 914-6, grey, 2.7L 325hp twin turbo Audi conversion

anything for a buck, some people dont know any better or dont care to look info up.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

The high NOx could also indicate a clogged EGR tube...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Dont think we have an EGR sytem on this one.

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2009 Mercedes ML320 Bluetec

How many miles? Have the valve stem seals ever been done?

High NOx can be caused by high compression caused by deposits in the combustion chambers. High unburned HC can be caused by the oil running down the valve stems. Time for a head job? It has twice made my aircare numbers a lot better.

Andrew
1989 190e 2.6