Since the car was originally registered as a gasser, requiring smog checks, Texas DMV may not be willing to change just because the car has a diesel engine in it now. However, most states have procedures that can be followed to register a one-off, which is essentially what you are creating. The trick is to find the DMV employee who will take the time to explain the paperwork procedure to you. My concern would be that some suspicious busybody would think you were doing this just to get out of the smog regs and were planning to swap a gasser engine back in as soon as you got the diesel registered. It would be worth a call to your local DMV just to see what the response is – let us know, please.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970