Honorable mentions for me is ford pickups with the triton V10, the VW taureg(sp?) with the tdi, and bringing up the rear is the diesel liberty.

All these vehicles have advantages, but i have the most trouble working on engineering compromise vehicles with large or unusual motors crammed in and the tradeoffs made to do that making for difficult repair procedures

Way at the top of the list is a military spec H1. Hardest vehicle i have ever tried to work on. Every single repair is a huge complex procedure.

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