[89 crew]

I have never heard of a factory 85 with the Isuzu diesel. In 85 the only factory diesel I'm aware of was the 6.2. One possibility is the 4500 w4's they were the same truck as the Isuzu 4500 series cab over. maybe they robed the engine out of that for the truck. Run the vin and it should tell you what the truck came with from gm.