Actually, in Maryland they do check the big rigs - even those passing through and registered in another state. The Hwy Patrol have smoke checkers they use on the exhaust pipe, checking for opacity of the smoke. They do spot checks and if they see a truck belching they can pull it over.

There is no testing on diesels under 10K lbs.

I would think they would test for opacity/soot since there were no NOX, CO, or other standards for diesels when our cars were built.

Keep us posted.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer