As far as I have known they have always had a diesel motor. I think the Chrysler prototype that AM General beat out had a BB V8 in it that was gas. But the AM General version always had a diesel. Though I remember reading somewhere along the way that the first protoype used a different engine and AMG switiched to the 6.2 just before going into production. But don't quote me on that as I'm going from memory of things I read long ago.
Edit: I see Westech beat me to the punch on posting so I see my memory was correct that it was a air cooled diesel first.
GM stopped offering a warranty to AM General on the 350 gasoline engine in the Hummer, truck was too heavy (so said GM) and AM General stopped offering it. They were a total fuel pig and prone to breakage. (Hummer owner since March 1993)
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