It would help if you gave your location? You can add it by clicking on the user CP at the top of the page. A while back someone did a 500SEC diesel conversion. I believe it was in Virginia and there weren't any problems mentioned. Most states don't require emissions checks for diesels or older (classic) cars.

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'83 240D (for sale)
'84 Harley FXRT
'85 300CD Turbo (Sold)
'85 500SEC (for sale)
'04 Dodge Dakota Quad
'06 Harley FLHTCI
'97 SL320