Given the great number of stories of Detroit Diesel engines experiencing overheating problems one might agree with you. I think, however, that by the time my "new" engines were manufactured in 1993 the DD engineers, perhaps under the ownership of Roger Penske, had corrected most of these problems. My engines and transmissions were purchased directly from DD and came with the fuel/transmission coolers on the downstream side of the raw water system just before the pipe to the showerhead. In the 25 years since then I have never had anything even close to an overheating condition. I think the reason is that the flow of raw cooling is so large that even in inland Florida waters the difference between the cooling water is only about 15 degrees higher between the inlet and the outlet of the heat exchanger.
As I thought about this I realized that the important thing about the DD 6-92 cooling system is that the maximum temperature should never be permitted to exceed about 190 degrees lest the rubber o-rings get cooked. Therefore the main heat exchanger, which cools both engine lubricating oil and the fresh water side of the coolant is by far the most important for engine life. So it gets the first crack at the coolest water. The fuel cooling probably isn't very critical at all. The transmission does develop some heat but not much compared to the engine (no fire). Because the engine and transmission are bolted together there is probably heat equalizing from one to the other and the designed coolers handle both. I check my transmission temps with my IR thermometer and have never noticed any great changes between engines.
I clean and check the main heat exchangers every few years. I just did not realize that the fuel/transmission coolers were an integral (and vulnerable) part of the entire engine cooling system. So I never checked them other than changing zincs. I also have never checked the turbo aftercooler because it is deep in the engine. Maybe I need to consider this, too.
Always something....
