I know its not a Buick but?

Agree on bad modulator....... should be less than a $20 part. takes 5 minutes to replace. They go without warning
and the smoke can be dramatic......!

 

Gonna try changing the PCV and transmission modulator. I thought the modulator needed to be adjusted as I did so on my 71 Buick to change the shift intervals. In that car it was original to the best of my knowledge but that trans was a 350 turbo, not 400.

 

They make both, you can get an adjustable modulator if you want to play with the shift points.

 

Just started the car up after I changed the PCV valve. Still no smoke, NOTHING! Bought a new transmission modulator which I will install shortly. I am stumped!